Fix go version number extraction

We really need to check to major.minor so 1.2.1 should be considered
1.2. while 1.32 should be considered 1.32 even though their aren't
double digit minor versions today I think it makes sense to consider
them.

This has been tested against the version strings

go version go1.3 darwin/amd64
go version go1.2.1 darwin/amd64
go version go1.2 darwin/amd64
go version go1.31 darwin/amd64

This version uses sed as default installed on OSX (afaik) @snej can you test on
your machine?

fixes #343