Don't fail silently if tests fail

Currently, `protractor:spec` returns with exit code 0 whether the protractor tests pass or not.

This can cause issues when using continuous integration, as the CI assumes that the rake task has completed successfully, and will move on to further steps in the process (such as migrations, deployments, etc.)

This passes the exit code of the protractor system call out through the rake task, so that exit code will be 1 if the tests fail.