Prevent tests running multiple times

When saving files with Vim on linux, tests are being run twice.

As seen in several Guard plugins, `run_on_changes` is running multiple times when saving files with Vim. This can be easily remedied by using `run_on_modifications` instead.

See:

guard/guard#297
guard/guard#495

The `run_on_additions` method is also being called when saving files with Vim, causing multiple invocations of the test(s). Because `run_on_modifications` is executed when a new file is created anyway, it should be unnecessary to run the tests explicitly from `run_on_additions`.