Add gopherjs to travis.yml

[EtienneBruines]

It'd be nice if we tested the build integrity of engo w.r.t. gopherjs as well.

Should be done after #247, though.

Something I found on the web:

language: go
go:
  - 1.5
addons:
  apt:
    packages:
      - nodejs
install:
  - go get -u github.com/gopherjs/gopherjs
script:
  - diff -u <(echo -n) <(gofmt -d ./)
  - gopherjs test

I can imagine we also have to use that script tag to define it. Not sure if we have to write some weird script to find all main files, though. (Travis now does that for us, using just Go)

[EtienneBruines]

Done in https://github.com/EngoEngine/engo/commit/e5a88d21b82a2fcab7048cf941b7bad02850c498