test-server: forward Karma/Protractor exit code

This ensures that `gulp test` fails with non-zero exit code
when tests failed.

This feature requires gulp 3.8.6 or newer, as the older versions did
not exit with non-zero code when a task failed.
Remove sandbox files from git.

The sandbox is a directory generated and modified by the tests.
Merge pull request #465 from strongloop/fix/test-server-exit-code

Fix test-server exit code, remove sandbox from git