rbenvのglobalが最新版にならない

macユーザです。
railsをインストールするためにrubyをアップデートし、tuby -vでしっかり2.5.3になっていることは確認できるのですが、rbenvの最新版(2.5.3)をインストールして rbenv global 2.5.3を実行しても、ずっとrbenv 1.1.1のままです。

rbenvを再インストールしても、すでに存在しますが続けますか？と画面に出てきます。

ちなみに $gem install railsをインストールしようとすると、

Building native extensions. This could take a while...
ERROR:  Error installing rails:
	ERROR: Failed to build gem native extension.

    current directory: /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/nokogiri-1.8.5/ext/nokogiri
/Users/johndoe/.rbenv/versions/2.5.3/bin/ruby -r ./siteconf20181121-62741-15hvv2e.rb extconf.rb
checking if the C compiler accepts ... yes
checking if the C compiler accepts -Wno-error=unused-command-line-argument-hard-error-in-future... no
Building nokogiri using packaged libraries.
Using mini_portile version 2.3.0
checking for iconv.h... yes
checking for gzdopen() in -lz... yes
checking for iconv... yes
************************************************************************
IMPORTANT NOTICE:

Building Nokogiri with a packaged version of libxml2-2.9.8
with the following patches applied:
	- 0001-Revert-Do-not-URI-escape-in-server-side-includes.patch
	- 0002-Fix-nullptr-deref-with-XPath-logic-ops.patch
	- 0003-Fix-infinite-loop-in-LZMA-decompression.patch

Team Nokogiri will keep on doing their best to provide security
updates in a timely manner, but if this is a concern for you and want
to use the system library instead; abort this installation process and
reinstall nokogiri as follows:

    gem install nokogiri -- --use-system-libraries
        [--with-xml2-config=/path/to/xml2-config]
        [--with-xslt-config=/path/to/xslt-config]

If you are using Bundler, tell it to use the option:

    bundle config build.nokogiri --use-system-libraries
    bundle install

Note, however, that nokogiri is not fully compatible with arbitrary
versions of libxml2 provided by OS/package vendors.
************************************************************************
Extracting libxml2-2.9.8.tar.gz into tmp/x86_64-apple-darwin18.2.0/ports/libxml2/2.9.8... OK
Running git apply with /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/nokogiri-1.8.5/patches/libxml2/0001-Revert-Do-not-URI-escape-in-server-side-includes.patch... OK
Running git apply with /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/nokogiri-1.8.5/patches/libxml2/0002-Fix-nullptr-deref-with-XPath-logic-ops.patch... OK
Running git apply with /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/nokogiri-1.8.5/patches/libxml2/0003-Fix-infinite-loop-in-LZMA-decompression.patch... OK
Running 'configure' for libxml2 2.9.8... OK
Running 'compile' for libxml2 2.9.8... ERROR, review '/Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/nokogiri-1.8.5/ext/nokogiri/tmp/x86_64-apple-darwin18.2.0/ports/libxml2/2.9.8/compile.log' to see what happened. Last lines are:
========================================================================
      _parseAndPrintFile in xmllint.o
  "_xmlXPathEval", referenced from:
      _doXPathQuery in xmllint.o
  "_xmlXPathFreeContext", referenced from:
      _doXPathQuery in xmllint.o
  "_xmlXPathFreeObject", referenced from:
      _doXPathQuery in xmllint.o
  "_xmlXPathIsInf", referenced from:
      _doXPathDump in xmllint.o
  "_xmlXPathIsNaN", referenced from:
      _doXPathDump in xmllint.o
  "_xmlXPathNewContext", referenced from:
      _doXPathQuery in xmllint.o
  "_xmlXPathOrderDocElems", referenced from:
      _parseAndPrintFile in xmllint.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [xmllint] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
========================================================================
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/Users/johndoe/.rbenv/versions/2.5.3/bin/$(RUBY_BASE_NAME)
	--help
	--clean
	--use-system-libraries
	--enable-static
	--disable-static
	--with-zlib-dir
	--without-zlib-dir
	--with-zlib-include
	--without-zlib-include=${zlib-dir}/include
	--with-zlib-lib
	--without-zlib-lib=${zlib-dir}/lib
	--enable-cross-build
	--disable-cross-build
/Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mini_portile2-2.3.0/lib/mini_portile2/mini_portile.rb:402:in `block in execute': Failed to complete compile task (RuntimeError)
	from /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mini_portile2-2.3.0/lib/mini_portile2/mini_portile.rb:373:in `chdir'
	from /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mini_portile2-2.3.0/lib/mini_portile2/mini_portile.rb:373:in `execute'
	from /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mini_portile2-2.3.0/lib/mini_portile2/mini_portile.rb:115:in `compile'
	from /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mini_portile2-2.3.0/lib/mini_portile2/mini_portile.rb:154:in `cook'
	from extconf.rb:365:in `block (2 levels) in process_recipe'
	from extconf.rb:257:in `block in chdir_for_build'
	from extconf.rb:256:in `chdir'
	from extconf.rb:256:in `chdir_for_build'
	from extconf.rb:364:in `block in process_recipe'
	from extconf.rb:262:in `tap'
	from extconf.rb:262:in `process_recipe'
	from extconf.rb:551:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/extensions/x86_64-darwin-18/2.5.0-static/nokogiri-1.8.5/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/nokogiri-1.8.5 for inspection.
Results logged to /Users/johndoe/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/extensions/x86_64-darwin-18/2.5.0-static/nokogiri-1.8.5/gem_make.out