Update GopherJS Playground to Go 1.6.

[dmitshur]

This is based on work in #41, which allows this entire step to be done
in a matter of seconds by simply running one command:

go generate github.com/gopherjs/gopherjs.github.io/playground

And it works even with multiple GOPATHs, and non-writeable GOROOT. \o/

It literally took me seconds to create this PR! Instead of what it used to take in the past (anywhere between 20-60 minutes depending on how recently I had done this manual operation).

(Note, this PR was generated with the work done in #41.)

[dmitshur]

Reviewed this myself, LGTM based on the fact that last such PR was reviewed with a comment "nothing to review, merge if it works" (see #39 (comment)). /cc @dominikh

Test plan, I used goexec 'http.ListenAndServe(":8080", http.FileServer(http.Dir(".")))' to start an HTTP server serving contents of playground directory, then visited localhost:8080 in browser, hard-cleared all cache, and tested a small Go program. It worked without errors: