FeaturesController: fix docs

The docs erroneously said to override `#permitted?`. The correct method
name is `may_manage_features?`.
gemspec: set signing_key conditionally

If ~/.ssh/gem-private_key.pem doesn't exist, don't try to set that
property. The travis build was breaking because Travis has no such
file and was trying to build the gem.
v1.5.3
Merge branch 'master' into james/global_whitelists