Install with homebrew ruby by default, if present

[nicksieger]

What

• Install brew gems with homebrew ruby by default, if homebrew ruby is installed
• Fix issue where homebrew superenv shims were interfering with Ruby's ability to compile native extensions.

URLs

Links to bug tickets or user stories.

QA Plan

• [x] Pull down this branch and run:

  PATH=./bin:$PATH brew gem install <gem>

• [x] Ensure that the gem installed successfully.
• [x] Inspect the binstub created for the gem and ensure that #!/usr/local/bin/ruby is at the top, indicating it is installed to use homebrew ruby.

[emmahsax]

[anfleene]

@nicksieger This worked fine for me:

[~/sportsengine/brew-gem]$ PATH=./bin:$PATH brew gem install opsicle                                           
==> Fetching opsicle from gem source
Fetching: opsicle-2.10.1.gem (100%)
Downloaded opsicle-2.10.1
Warning: Cannot verify integrity of opsicle-2.10.1.gem
A checksum was not provided for this resource
For your reference the SHA256 is: 36490cc2b2fc68bfdc7cb4afa0dfb879c3978bc8853770955d08b40c71afccae
==> /usr/local/bin/gem install /Users/anfleene/Library/Caches/Homebrew/opsicle-2.10.1.gem --no-ri --no-rdoc --no-wrapper --no-user-install --install-dir /usr
🍺  /usr/local/Cellar/gem-opsicle/2.10.1: 1,271 files, 14.6MB, built in 18 seconds
PATH=./bin:$PATH brew gem install opsicle  12.81s user 4.71s system 80% cpu 21.761 total

[emmahsax]

[production-status-check[bot]]

:octocat: Has QA approval