add if statement to check if Raven object has been instantiated

[ZacharyKearns]

Description of changes

Checks to make sure that the Raven object exists before calling any methods on it.

Issue Resolved

Fixes #955

[jjhampton]

@ZacharyKearns Wrapping the inline <script> tag in an if-statement prevents the Raven-related console errors, but that's because the if-statement will never be true. So this also prevents the Raven.config method from ever being called. That's because when this script tag runs, Raven hasn't been loaded because it's loaded asynchronously w/ a defer attribute on line 37 in the head (this was introduced as a performance improvement in PR #932).

Removing the defer would probably fix this (and there would be no need for the if-statement either, since Raven would have already been loaded synchronously from CDN).

@kylemh I see three options here:

1. Remove the defer from the Raven CDN script for now (simple fix)
2. Explore one of the more complicated ways to load Raven asynchronously (Asynchronous Loading and Capturing Errors getsentry/sentry-javascript#169 outlines some of these)
3. Remove Raven completely - I've never used it, and I don't know of any of the current maintainers who have, either. YAGNI

Thoughts?

[jjhampton]

See inline comments - console errors are cleared, but Raven config isn't being set up w/ this solution.

[kylemh]

Fixed in #980