Fix Travis builds

Update ease to 0.1.3

Update requirements.

Fixed NoneType value for days_early_for_beta and added a unit test.

TNL-2139

Update pytz to 2015.2

pytz changed their versioning scheme from using letters to using
numbers. The old scheme is screwing up new versions of pip.

Better i18n for due/available time format

Fixed student can't view their submission after recieving grades.

TNL-2000

There are various different such format strings here. But they all result in the same format. So combine them into only two.

Fix a placeholder's translation

'Preview in Latex' was not working in ORA.

This is shown when no text is entered in a response textbox.

TNL-1755

Update NLTK to 2.0.5

Added test for TNL-1758's fix.

TNL-1827

Added beta tester functionality.

TNL-1736

Dispaly rubric-option label instead of name

Fixed training example answer format for old submissions.

TNL-1758

Update assessments format when serializing.

Allow file upload backends other than S3. All tests are passing - merging.

Fix mount directory bug in Vagrantfile

Update translations.

Adds support for multiple prompts in problems.

Disable adding/removing prompts after release.

We would like to support open assessment file uploads for open-edx
installs without access to S3, e.g: to the local fileystem. To that end,
we propose to abstract away some of the logic behind the file upload
backends so that other backends may be implemented.

In this proposed implementation, the uploaded files are stored in the
following folder structure:

        root/bucket/prefix/
                filename.jpg/
                        content
                        metadata.json

Note that a couple new settings need to be defined in order to use this
feature; a new urls.py file must also be included.