Ensure that if artifact is published, we overwrite default checksums.

Fixes # 1233

* Add a new "shim" for RepositoryResolvers that modifies the "put"
  method so that it will ignore the overwrite flag for checksums.
Remove JDK7 features from Resolver shim.
Merge pull request #1232 from sbt/wip/fix-1156

Ensure that if artifact is published, we overwrite default checksums.