Fix mocha version

[ogonkov]

Summary

Proposed change:

Since mocha @ 8.x it's not compatible with now deprecated mocha-phantomjs-core. As temporary solution this PR is fixing mocha version to latest release in 7.x upstream.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[codecov-commenter]

Codecov Report

Merging #1292 into master will decrease coverage by 0.04%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master    #1292      +/-   ##
==========================================
- Coverage   89.60%   89.55%   -0.05%     
==========================================
  Files          22       22              
  Lines        3020     3025       +5     
==========================================
+ Hits         2706     2709       +3     
- Misses        314      316       +2     

Impacted Files	Coverage Δ
nunjucks/src/node-loaders.js	65.67% <0.00%> (-2.99%)	⬇️
nunjucks/src/filters.js	96.18% <100.00%> (+0.06%)	⬆️

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[mroderick]

This PR looks good to me. Is anything holding it back from being merged?

[ogonkov]

See @fdintino comment #1279 (comment)

I don't know any other maintainer that is active in this repo :(