Run on modifications instead of changes

Due to the way vim saves files the corresponding tests
are run three times. To prevent this use run_on_mondifications
instead of run_on_changes.

See also:
* https://github.com/guard/guard/issues/495
* https://github.com/modeset/guard-teaspoon/issues/18
Merge pull request #49 from sch0rsch/run-on-modifications-instead-of-changes

Run on modifications instead of changes