Chore migrate on latest babel and ava

[alexander-akait]

No description provided.

[codecov-io]

Codecov Report

Merging #772 into master will decrease coverage by 0.03%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
- Coverage   98.69%   98.66%   -0.04%     
==========================================
  Files         110      116       +6     
  Lines        3061     3140      +79     
==========================================
+ Hits         3021     3098      +77     
- Misses         40       42       +2

Impacted Files	Coverage Δ
packages/css-size/src/index.js	100% <100%> (ø)	⬆️
babel.config.js	80% <80%> (ø)
...kages/postcss-merge-longhand/src/lib/minifyTrbl.js	100% <0%> (ø)	⬆️
...postcss-discard-comments/src/lib/commentRemover.js	100% <0%> (ø)	⬆️
...ckages/stylehacks/src/plugins/leadingUnderscore.js	100% <0%> (ø)	⬆️
...ages/postcss-normalize-display-values/src/index.js	100% <0%> (ø)	⬆️
packages/postcss-merge-idents/src/index.js	100% <0%> (ø)	⬆️
packages/postcss-unique-selectors/src/index.js	100% <0%> (ø)	⬆️
...ages/postcss-merge-longhand/src/lib/hasAllProps.js	100% <0%> (ø)	⬆️
packages/postcss-reduce-initial/src/index.js	100% <0%> (ø)	⬆️
... and 62 more

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[alexander-akait]

lerna doesn't work fine with node@6 so i disable CI on node@6, but our babel setup for 6.9.0 so all work fine

[alexander-akait]

We need migrate on jest, ava is very slow on windows, let's merge this as is because test passed for linux and starts to migrate on jest