Ruby

So, why would you keep seeing ruby 2.3.7p456 instead of the selected version (as shown by rbenv global)?

The reason is that you have an out of date version of ruby-build? Run: 

cd /Users/christopher/.rbenv/plugins/ruby-build && git pull && cd -
ChristophersMBP:bin christopher$ rbenv install 2.6.1
ruby-build: definition not found: 2.6.1

See all available versions with `rbenv install --list'.

If the version you need is missing, try upgrading ruby-build:

  cd /Users/christopher/.rbenv/plugins/ruby-build && git pull && cd -
ChristophersMBP:bin christopher$ cd /Users/christopher/.rbenv/plugins/ruby-build && git pull && cd -
remote: Enumerating objects: 280, done.
remote: Counting objects: 100% (280/280), done.
remote: Compressing objects: 100% (17/17), done.
remote: Total 423 (delta 261), reused 276 (delta 259), pack-reused 143
Receiving objects: 100% (423/423), 110.66 KiB | 7.90 MiB/s, done.
Resolving deltas: 100% (312/312), completed with 64 local objects.
From https://github.com/rbenv/ruby-build
   648fc99..ca85eba  master     -> origin/master
 * [new tag]         v20190423  -> v20190423
 * [new tag]         v20181207  -> v20181207
 * [new tag]         v20181225  -> v20181225
 * [new tag]         v20190130  -> v20190130
 * [new tag]         v20190314  -> v20190314
 * [new tag]         v20190320  -> v20190320
 * [new tag]         v20190401  -> v20190401
Updating 648fc99..ca85eba
Fast-forward
 bin/ruby-build

No, the actual issue is that your PATH was wrong. You were missing the SHIMs directory: 

export PATH="$HOME/.rbenv/shims:$HOME/.rbenv/bin:$PATH"