Prevent polling just because the adapter gem was already required

Listen::Adapter.load_dependend_adapter returns true even if already
loaded.  Otherwise, if (for example) `gem "rb-fsevent"` already appears
in a Gemfile or some other gem requires the adapter_gem before we do,
Listen will always fall back to polling.
Merge pull request #118 from nilbus/already-required-gem-forces-polling

Prevent polling just because the adapter gem was already required