Upgrade to bouncycastle 1.51 and stop using deprecated methods

[benmccann]

sbt-pgp blows up when other packages (such as Play 2.3.3) pull in newer versions of bouncycastle. This is because sbt-pgp is using deprecated methods that were removed in 1.51.

E.g. instead of using PGPKeyPair we should be using BcPGPKeyPair or JcaPGPKeyPair as appropriate.

This is why it's bad to ignore deprecation warnings ;-)

[seratch]

Same issue? playframework/playframework#3327 (comment)