Add dryRun mode switch in PushOp

dryRun mode for push is useful when implementing multi step release process with dryRun mode available in every command. It would be also useful to have an optional message informing what would have been performed, but probably jgit is a better place to implement that.

By the way I renamed duplicated method in tests (it failed to compile in Idea, but worked fine in Gradle) and updated Gradle wrapper (due to an issue with importing Gradle 2.1 projects into Idea 14 when using OpenJDK).