Image listing

[pp-mo]

Added a file to contain a listing of the all v4 image hash files in it.

This provides a mechanism to avoid the iris 'test_image_json' calling the GitHub web api,
as the HitHub Api appears to be fragile and unreliable (at least in the way we were calling it).
Instead, the iris test will in future download the listings file and check that the required hashes are all in it. See SciTools/iris#3308 or possibly SciTools/iris#3309

Here, also :

• added a util to refesh/recreate the listing file.
• added a new test to check that the listing file matches the v4 directory, so we won't merge PRs into this repo until the listings file is right.
  (if the test fails, it prompts you to run the update util)

ALSO:

• modded .travis.json to get tests working in this repo again, which stopped working ages ago (!whoops)
• added a .gitignore as it seems we need one