Set go version to 1.3.3

This bumps the version from the current default version of 1.2.2.

Go 1.3 brings several performance improvements.  See
https://golang.org/doc/go1.3 for full details.

In general, it's good to keep up to date.
Merge pull request #75 from alphagov/set_go_version

Set go version to 1.3.3