Update bouncycastle license reference

LICENSE file says 'Bouncy Castle JDK 1.6 Provider' Copyright (c) 2000 -
2013, but should probably say 'Bouncy Castle JDK 1.5 Provider' Copyright
(c) 2000 - 2015 based on changes to these dependencies in NiFi 0.5.0. This
doesn't seem significant enough to hold up the release as long as we fix it
in good faith on the master branch.