Upgrade RBTools to 0.5.5.

This pins us to a stable released version of RBTools, with a number of bugfixes [1,2].

[1] https://github.com/reviewboard/rbtools/commits/release-0.5.5
[2] https://github.com/reviewboard/rbtools/commit/c9ce18becce5ef00fa5e70d51511ad43aab03601

Testing Done:
Posted this review, verified upgrade works.

Reviewed at https://reviews.apache.org/r/17464/