check if go version is 1.2 or above

Check if the go version is above 1.2, instead of matching hard against
1.2. Since 1.3 is released and the tests seem to pass I think it should
be possible to build with 1.3 as well. This is needed to build with
homebrew as well since go 1.3 is the default version there, see
https://github.com/Homebrew/homebrew/pull/29736

Fixes #343