Upgrade RuboCop to avoid blowing away YAML engine.

RuboCop was permanently modifying the YAML engine Ruby was using.
0.25 does not suffer from this problem.

Closes #150
Less strict error message checking.

Depending on the version of scss-lint your machine is running,
the message slightly changes.
Merge branch 'master' into fix_manually_passing_files_to_runner

Conflicts:
	test/unit/plugins/pre_commit/checks/scss_lint_test.rb