Upgrade to retryable 1.3.2, which fixes a few issues. Fix gemspec and Gemfile.

Recent updates to retryable fix a few issues:
* The last exception is yielded to the `retryable` block (as has been claimed by docs for a while)
* By default, `StandardError` is rescued rather than `Exception`.

Also, I removed what appeared to be redundant dependencies from the .gemspec file and fixed the Gemfile to defer to the gemspec.