PGP: Dont write version header by default to armored messages

By telling the recipients which version of Bouncy Castle has been used, it is much easier to find exported keys in the Internet which have been generated by a potentially broken version of the library.
Writing a version header should be totally optional.
see also http://blog.cryptographyengineering.com/2014/08/whats-matter-with-pgp.html
and https://mailman.boum.org/pipermail/tails-dev/2013-August/003435.html