rake:release should guard against both uncommitted and unstaged changes
support setting boolean environment variables

These were previously stored as a string if they are derived from the
environment or configuration files. Rather than altering each of the checks,
we can select a list of settings to convert to boolean values when they are
accessed.
abstract Marshal.load failures
Merge pull request #2088 from benmoss/rake_release_uncommitted

rake:release should notify of both unstaged and uncommitted changes

Conflicts:
	spec/bundler/gem_helper_spec.rb
Merge pull request #2144 from brettporter/bool-settings

support setting boolean environment variables
Remove default version on `gem` method calls since it was never being evaluated
stop shelling out to which
leave .bat gem bins too

fixes #1495