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.