upgrade gradle, move to 2.5.0

@alonle @liorhar 
bumps the plugin to gradle 1.12 & proto to 2.5.0 
I couldn't get the "applyBinary" to actually fetch the execs so I placed them manually 
This is pretty much taking the original plugins current state & adding our changes
 1. bring exec/bin as depenedcy  (doesn't work)
 1. remove compile java dependency on generateProto
 1. configurable ourput dir