support caching for gemfiles in subdirectories, fixes travis-ci/travis-ci#2975
update travis-support
bundler path is interpreted as relative to gemfile, casher path is interpreted as relative to cwd
Merge branch 'master' into rkh-gemfile-subdirectory

Conflicts:
	lib/travis/build/script/shared/bundler.rb
	spec/build/script/directory_cache_spec.rb