Added mkdir -p for GOPATH/src/github.com
Added mkdir -p for GOPATH/src/github.com

With that, after dotfiles is instaled, the `g` alias will work out of
the box. Right now it won't because the src/github.com folder might not
exist yet.

dennari/dotfiles-new,mattmahn/dotfiles,caarlos0/dotfiles,SillyMoo/dotfiles,mattmahn/dotfiles