bump Go to 1.3.1

I've runned all tests and compiled binary with 1.3.1.
Just to clarify, since there might have been some confusion - I mean that I'm +1 on keeping only the latest version of both major releases here.  If a distro has 1.3 packaged already, it's easy for them to get 1.3.1 (in most cases).  Going from 1.2 to 1.3 (for example in Fedora 19 or Fedora 20 which are now "stable" and thus much more rigid) is challenging from a distro policy POV, so even if performance is degraded for Go 1.2-compilers (and only people compiling against Go 1.3 get the real shinies and speedups), we ought to keep that retrocompat as long as we can so that people on those older distros can update to a newer Docker even if they won't get quite as speedy of a Docker (since they will then get other not-speed-related bugfixes too).