chore(travis): run the build twice (BS and SL)

Temporarily run the each job twice:
- using BrowserStack
- using SauceLabs

Author: vojtajina

.travis.yml

@@ -11,6 +11,9 @@ env:
     - JOB=unit BROWSER_PROVIDER=saucelabs
     - JOB=e2e TEST_TARGET=jqlite BROWSER_PROVIDER=saucelabs
     - JOB=e2e TEST_TARGET=jquery BROWSER_PROVIDER=saucelabs
+    - JOB=unit BROWSER_PROVIDER=browserstack
+    - JOB=e2e TEST_TARGET=jqlite BROWSER_PROVIDER=browserstack
+    - JOB=e2e TEST_TARGET=jquery BROWSER_PROVIDER=browserstack
   global:
     - SAUCE_USERNAME=angular-ci
     - SAUCE_ACCESS_KEY=9b988f434ff8-fbca-8aa4-4ae3-35442987