Update gatsby in /docs from 1.9.155 to 1.9.157

[dependencies]

Dependencies.io has updated gatsby (a npm dependency in /docs) from "1.9.155" to "1.9.157".

1.9.157

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1.9.156

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[codecov]

Codecov Report

Merging #2763 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2763   +/-   ##
=======================================
  Coverage   35.73%   35.73%           
=======================================
  Files         433      433           
  Lines        9430     9430           
  Branches     1006     1002    -4     
=======================================
  Hits         3370     3370           
- Misses       5362     5403   +41     
+ Partials      698      657   -41

Impacted Files	Coverage Δ
app/vue/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/polymer/src/server/utils.js	0% <0%> (-53.58%)	⬇️
addons/knobs/src/angular/helpers.js	83.33% <0%> (ø)	⬆️
addons/knobs/src/react/WrapStory.js	49.42% <0%> (ø)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
addons/actions/src/lib/types/function/index.js	77.77% <0%> (ø)	⬆️
addons/knobs/src/components/types/Date/index.js	23.52% <0%> (ø)	⬆️
addons/actions/src/lib/retrocycle.js	54.54% <0%> (ø)	⬆️
addons/knobs/src/components/types/Boolean.js	11.62% <0%> (ø)	⬆️
addons/storyshots/src/vue/renderTree.js	60% <0%> (ø)	⬆️
... and 48 more

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