Add repo2docker version to the Tale manifest. Fixes #388

[Xarthisius]

No description provided.

[codecov]

Codecov Report

Merging #410 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   90.65%   90.66%   +0.01%     
==========================================
  Files          47       47              
  Lines        3510     3515       +5     
==========================================
+ Hits         3182     3187       +5     
  Misses        328      328

Impacted Files	Coverage Δ
server/lib/manifest.py	90% <100%> (+0.28%)	⬆️

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[ThomasThelen]

To Test:

1. Checkout and deploy
2. Launch a Tale instance
3. Export it
4. Examine the Tale manifest and look for schema:hasPart
5. Create (but don't launch) a Tale
6. Export & examine manifest again