Interp test fixes

This fixes all the outstanding test failures 
 -- *except* the problem one addressed by https://github.com/cpelley/iris/pull/14
So it won't pass, but check out the Travis log to confirm that is the only remaining problem.

To do this, I also cherry-picked the pep8 v1.5 fixes from https://github.com/SciTools/iris/pull/1080.
That makes for a lot of files changed, but the individual commits should be comprehensible.