Remove obselete spec.

The worker class is no longer concerned with retryable exceptions.
Remove redundant boot environment lines from the failure backtraces.

Failures take up a large amount of redis memory due to the size of the backtrace. This filters out the redundant lines that form the beginning of the backtrace for every failure.
Use `.` in place of the current working directory in failure backtraces.

This will save even more memory.
Merge pull request #97 from seomoz/make_failures_use_less_memory

Make failures use less memory