Change the paths to the back arrow and spideroak button so that they once
again reliably appear.

I am mystified why this change should suddenly be necessary.

I confirmed that the problem exists with a fresh clone, and don't know what
change caused the existing paths to become incorrect.
Merge pull request #90 from SpiderOak/resurrect-back-arrow-and-icon

Change the paths to the back arrow and spideroak button so they appear.