Deprecate FeaturesHelper#error_messages_for
gemspec: correct gem files
v1.6.1
Remove FeaturesHelper#error_messages_for
v1.7.0
Use a gem-specific .pem file for signing

This prevents `Gem::Security::Exception`s for those who have a different
private key in the same location on their machine.

Closes #65
do not hit the database for updated at after current expired once