Flatten paths in case a directory is given

This will allow directories to passed in the guard file.

Like so:

````ruby
watch(%r{test/javascripts/.+_helper\.(js|coffee|js\.coffee)}) { 'test/javascripts' }
````

Otherwise it will throw this error "TypeError: no implicit conversion of Array into String".

Also, `mock` has been deprecated in favor of `double` so I went ahead and made those changes.