Upgrade golang package to 1.2.1

This takes the Ubuntu Trusty packaging for 1.2.1 and backports it to Precise.

Updating should not cause any issues for existing code because of the [promise of compatibility](http://golang.org/doc/go1compat) that Go gives for all 1.x releases.  

This release brings some new features that are useful to us:
* performance improvements (including a 30% boost for JSON encoding)
* `go get -t` option to simplify fetching test dependencies.
* test coverage tool

Full release notes: http://golang.org/doc/go1.2