Check if constant exists before defining

We are getting a lot of errors after upgrading to the latest version. The problem is that PHP cannot redefine a constant, so I added a check to see if it already exists before defining again.

Example error:
 [exec] 5) ApiControllerTest::testFeedbackSpecialChars
     [exec] PHPUnit_Framework_Exception: PHP Notice:  Constant ROLLBAR_INCLUDED_ERRNO_BITMASK already defined in /var/lib/jenkins/jobs/ci_development/workspace/protected/vendors/rollbar/rollbar.php on line 81
     [exec] PHP Stack trace: