Run integration tests on Travis

[daniel-j-h]

We should run

• the model conversion tool,
• the extract tool, and
• the semcode tool

on Travis on either dummy input or cached weights and videos to make sure the tools work.

The changeset here is a first step in this direction. WIP.

[daniel-j-h]

• [x] downloading official pkl blob and caching them in the Travis cache set up
• [x] model conversion tool hooked up

Working for a single model right now; add all of them!

Next: hook up extract and semcode tools.

[daniel-j-h]

• [x] convert
• [x] extract
• [x] semcode

Tools hooked up; caching pkl blobs so we don't have to download them on every commit.