chore(travis): enable both SauceLabs and BrowserStack

Setting env var `BROWSER_PROVIDER` to `browserstack` or `saucelabs`
determines which browser provider will be used.

Switch unit tests (`JOB=unit`) to use BS.
Does not affect e2e builds (`JOB=e2e`).

Switch to Karma with Socket.io 1.x, which solves some issues(*) with BS.
Thus removing `config.transports` as it is not used anymore
(Socket.io 1.x starts with polling and tries to upgrade if available).

(*) folks from BS were fiddling with socket.io configuration to get it stable.
See https://github.com/dhimil/karma/commit/4c04011850bf66a8a7556cd76ad662c568399481
This is not necessary with Socket.io 1.x.