Gradle: Upload signed Spongy Castle artifacts to Sonatype OSS Nexus

http://yennicktrevels.com/blog/2013/10/11/automated-gradle-project-deployment-to-sonatype-oss-repository/

Using a 'spongycastle' subgroup (ie com.madgag.spongycastle) to give a useful
grouping, apparently this is legit:

https://docs.sonatype.org/display/Repository/Choosing+your+Coordinates

Also guard against missing gradle.properties for travis, which doesn't have
to do signing or uploading to sonatype.
Fix provider mapping for KeyStore - should be "BC" not "SC"

I think "BC" here is a KeyStore format, not a provider.

Would be nice to not have to make this correction, but without
code changes, I think it wouldn't be easy to write a distingushing
ssed expression.
Disable test that attempts to deserialise org.bouncycastle class