Remove Version#to_a alias

Exposing this as "to_a" was a mistake, versions are not arrays and it
causes incorrect behavior when splatted or using Kernel#Array(). Use the
more correct name "tokens" instead.
Pass these as strings, they get converted anyway
Simplify setting MACOS_VERSION
Run tests under actual OS version
brew-test-bot: run Homebrew tests every time.
brew-test-bot: handle Jenkins pull request plugin.
brew-test-bot: fix testing pull request handling.
brew-test-bot: further tweak pull request handling.
Avoid unnecessary float to string conversion
Add minimum OS X version requirement

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
Use minimum OS X requirement