Using the default GOROOT if not set

Hi Luca, 

It seems that setting `$GOROOT` is only recommended if you have installed `go` to a non-standard location, ie. not in `/usr/local/go`

http://dave.cheney.net/2013/06/14/you-dont-need-to-set-goroot-really

There might be a nicer way of writing the below but it sorted out the syntax highlighting for me :)