ruby-build 20130907

Closes #22370.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
brew-test-bot: run brew uses.
nexus 2.6.2-01

Closes #22369.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
nexus: use version variable.
nexus: replace patch with inreplace.
nexus: remove caveats, shorten plist_options.
qt, qt5: cleanup hacky, unused, misnamed options.

Closes #22321.
Closes #22332.
Optionally use git to keep brew etc versioned.

Still in alpha state.
Handles defaults and merging changes with new versions.
Enable by setting the HOMEBREW_GIT_ETC environment variable.

Closes #15751.
Closes #17713.