guard/guard — pull request #531 by uk-ar (2013-12-12 21:10:56+00:00)
Title: Callback for run_on_modifications is not working.

I tried to use callback for run_on_modifications with README example(https://github.com/guard/guard#callback). But following simple Guardfile is not working.
```
guard :rspec do
watch(%r{^spec/.+_spec\.rb$})

callback(:run_on_modifications_begin) { `mate .` }
end
```
So I added spec for this issue.