Change rb-fsevent dependency to work on OS X 10.6-10.8
Change rb-fsevent dependency to work on OS X 10.6-10.8

Change `rb-fsevent` dependency from `>= 0.9.7` to `>= 0.9.4`, to enable gem
to work on OS X 10.6 through 10.8.

Needed as `rb-fsevent > 0.9.4` no longer supports OS X 10.6 through 10.8.

Tested on OS X 10.8 via use with `guard-rspec` gem.

Fixes #392

guard/listen