Add [publish binary] message to release task

With these Makefile changes, merging master into osx-node-pre-gyp during the
'make release task' will:

- always force a commit
- add a "[publish binary]" message to the commit to tell Travis to build and
publish a new binary
Merge pull request #321 from hybridgroup/makefile-merge-commit

Add [publish binary] message to release task