Fix homebrew script - checkout master, not origin.

[ci skip]
Don't load RSpec nor Bundler during Hombrew install

When being installed by Homebrew we don't have rspec nor bundler
available, so don't load them.
Look for Homebrew constant instead
Trying to find an ENV var we can use.
Prepare for 1.5.0 release

Split out the history to reflect the 1.4.0 release, which was out for
quite a while.

[ci skip]
Bump to v1.5.0