Fix test-server exit code, remove sandbox from git

test-server: forward Karma/Protractor exit code. This ensures that `gulp test` fails with non-zero exit code
when tests failed. Note that this feature requires gulp 3.8.6 or newer, as the older versions did
not exit with non-zero code when a task failed. See https://github.com/gulpjs/gulp/issues/113 for details.

Remove sandbox files from git. The sandbox is a directory generated and modified by the tests.

Close https://github.com/strongloop/loopback/issues/700

/cc @seanbrookes FYI, there isn't much to review here.