Disable pattern matching in val syntax in sbt 0.13.7.

Fixes #1661 or at least works around it for now.


Review by @eed3si9n cc> @jaceklaskowski 
@xuwei-k said isn't there a better API than using toString https://twitter.com/xuwei_k/status/520639740274417664. We could check that the last parameter in `ValDef` and `DefDef` are not `EmptyTree` and get the same effect as checking for `=`.