More descriptive gem description

[albertyw]

As @justinucd mentioned, the LaunchDarkly SDK is hard to search for on rubygems. I think replacing the gem description in https://github.com/launchdarkly/ruby-client/blob/master/ldclient-rb.gemspec#L12 will make the gem easier to find in https://rubygems.org/gems/ldclient-rb. Perhaps wording like Official LaunchDarkly SDK for Feature Flags?