Fix nltk dependency

[spoeschel]

Use the last version that supports Python 2.7.

Fixes #533.

[nigelmegitt]

@spoeschel the build fails, but not obviously because of the change to a specified version of nltk. I think we may need to resolve some other issue first, but I'd be happy to do it in the same pull request. What do you think?

[spoeschel]

With version 3.2.1 of Pytest-BDD, the error doesn't occur and the (non-skipped) tests all pass. That version is actually the same one that automatically was used by the most recent passing Travis builds.

The two errors that show up with a recent version are caused by the comments that are present at the line ends of some feature example tables (example). While moving the comments to a separate line fixes the errors, the tests still don't pass. Hence this seems to be a larger issue.

So I think the easiest solution would be:

• add an upper bound (<3.3.0) for Pytest-BDD as well, for the Python 2 branch (as part of this MR is fine for me)
• open a separate issue in order to address the test errors with Pytest-BDD 3.3.0+ for the Python 3 branch in the future

Would this be OK for you?

[nigelmegitt]

Ah, that problem. We've already fixed that in the BBC python3 fork. Yes, happy to pin the Pytest-BDD version for the python2 branch to make a release that works.

[coveralls]

Coverage remained the same at 85.104% when pulling 6e5090300c541ea5f011784f9b46fe7bf66dc6b1 on issue-533-fix-nltk-dependency into a8d18aba2468460c63bd2e84507b68658303a0a3 on master.

[spoeschel]

Thanks, @nigelmegitt! I will have a look at possible further maintenance fixes for a 2.1.3 release.