Include JDK version and java target version in cache identifier

[spockz]

Currently caches, e.g. with stash, are scoped to the project name and the major scala version. This should probably be the full scala compiler version and include the version of the JDK, as well as the java target version.

[romanowski]

I can see your point why we would like to scope it with full compiler version and jdk version but the only reason why have automatic scoping by scala binary version is to support cross compilation so you can invoke +stash and +stashApply. I think I can make those default prefix configurable.

[spockz]

Having it configurable would be optimal. Then everyone can determine their own.