Upgrade to bouncycastle 1.51

This fixes the warning being printed out with 1.50

    [WARNING] bcprov-jdk15on-1.50.jar, bcpg-jdk15on-1.50.jar define 3 overlappping classes: 
    [WARNING]   - org.bouncycastle.apache.bzip2.CBZip2InputStream
    [WARNING]   - org.bouncycastle.apache.bzip2.CRC
    [WARNING]   - org.bouncycastle.apache.bzip2.BZip2Constants
    [WARNING] maven-shade-plugin has detected that some .class files
    [WARNING] are present in two or more JARs. When this happens, only
    [WARNING] one single version of the class is copied in the uberjar.
    [WARNING] Usually this is not harmful and you can skeep these
    [WARNING] warnings, otherwise try to manually exclude artifacts
    [WARNING] based on mvn dependency:tree -Ddetail=true and the above
    [WARNING] output
    [WARNING] See http://docs.codehaus.org/display/MAVENUSER/Shade+Plugin