Move feature logic to Repository

Repository should be responsible for knowing whether or not
a given feature about matrix expansion is active.

This implementation is a bit awkward, having `respond_to?(:repository)`
in `Build::Matrix#matrix_config`.
Merge branch 'sf-hiro-bugfix' of github.com:travis-ci/travis-core into sf-hiro-bugfix

Conflicts:
	spec/travis/model/build/matrix_spec.rb