Video-js and VTT.js upgrade

[benjaoming]

Summary

Fixes #5347

It became a bit of an assorted fixes and improvements PR, so here's an overview:

• Makes srt serving work with DEBUG=True and runserver
• Removed the minimal content pack fetching from assets make target.
• Added minimal content pack as a sort of test data content pack, we should rebuild a much smaller one though
• Removed unused and unmaintained .travis.yml

[codecov-io]

Current coverage is 51.02% (diff: 66.66%)

Merging #5349 into develop will decrease coverage by 0.06%

@@            develop      #5349   diff @@
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  Files           142        142          
  Lines          7482       7485     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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- Hits           3822       3819     -3   
- Misses         3660       3666     +6   
  Partials          0          0          

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[benjaoming]

Interestingly, this PR starts work on removing the minimal English content pack so also addresses #5318

[benjaoming]

On another note, the current patched git forks of video.js + vtt.js require the build systems to be able to fetch stuff in package.json with git -- no problem on Linux, but could cause issues on Mac / Windows build systems.. I can fix this if necessary @mrpau-eduard @mrpau-richard so let me know if make assets or npm install fails in your installer scripts after this merge.