Add libgconf-2-4 to Travis to get Electron running in tests

[astronomersiva]

No description provided.

[stefanpenner]

oooo, this may fix #1350

[astronomersiva]

Yes! The issue in #1350 is fixed by this.

Still...Sauce Labs' tests are failing with The environment you requested was unavailable. and AppVeyor seems to have a real failure:

dev mode app in interactive mode allows to restart a run:
     Uncaught TypeError: Cannot read property 'pid' of null
      at kill (lib\utils\process.js:122:33)
      at Process.onKillTimeout (lib\utils\process.js:36:5)

[stefanpenner]

@astronomersiva this PR does seem to fix the issue that it sets out to fix. I'm going to merge, we can explore the sauce labs problem in a separate PR.