disable BrowserStack tests due to flakiness

Running "browserstack_runner" task
Using config: /home/travis/build/twbs/bootstrap/browserstack.json
Launching server on port: 8888
Downloading tunnel jar to `/home/travis/build/twbs/bootstrap/node_modules/browserstack-runner/lib/BrowserStackTunnel.jar`
Launching tunnel
Error: Invalid or corrupt jarfile /home/travis/build/twbs/bootstrap/node_modules/browserstack-runner/lib/BrowserStackTunnel.jar
[Runner alert] Tunnel launch timeout
Exiting
Non existent tunnel

.travis.yml

@@ -26,6 +26,6 @@ env:
     - TWBS_TEST=core
     - TWBS_TEST=validate-html
     - TWBS_TEST=sauce-js-unit
-    - TWBS_TEST=browserstack-js-unit
+#    - TWBS_TEST=browserstack-js-unit
 matrix:
   fast_finish: true