[WIP] `foodcritic` task should not be run, if no cookbook is modified.

`foodcritic` task should detect any cookbooks' modification,
and only then execute `foodcritic` command should be executed.
`foodcritic` task should be run, when cookbooks are modified.

`foodcritic` task should be run,
when any cookbooks are modified.

This commit implements the above specification.
Merge pull request #150 from kazu634/foodcritic-task-modification

`foodcritic` task should not be run, if no cookbook is modified.