SF-1272: Pin version of CircleCI Node image to 11.6.0

[jmbtutor]

This pins the version of node to 11.6.0 and yarn 1.12.3. node 11.7.0 or yarn 1.13.0 causes nyc to report 0% coverage.

istanbuljs/nyc#921
yarnpkg/yarn#6746

circleci/node:11.6.0
Digest: sha256:1a3a10a092bdfa3d92d48850818a5526b5770a58592156f6f8e6a5f7d946ac43
Created 2019-01-18T00:14:29.784667795Z
NODE_VERSION=11.6.0
YARN_VERSION=1.12.3

[JoeyDeol]

Looks good, we should keep an eye out for when the next version of yarn addresses this bug.

[codecov-io]

Codecov Report

Merging #64 into master will increase coverage by 93.96%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #64       +/-   ##
===========================================
+ Coverage       0%   93.96%   +93.96%     
===========================================
  Files          35      206      +171     
  Lines         287     5688     +5401     
  Branches       51      709      +658     
===========================================
+ Hits            0     5345     +5345     
+ Misses        287      250       -37     
- Partials        0       93       +93

Impacted Files	Coverage Δ
...ackages/@storefront/core/src/core/service/index.ts	100% <0%> (ø)
packages/@storefront/structure/src/pages/index.ts	100% <0%> (ø)
...front/flux-capacitor/src/core/sagas/refinements.ts	96.87% <0%> (ø)
...ckages/@storefront/core/src/core/tag/decorators.ts	82.5% <0%> (ø)
...ront/core/src/core/url-beautifier/search/parser.ts	98.57% <0%> (ø)
...ront/products/src/swatch-variant-controls/index.ts	100% <0%> (ø)
packages/@storefront/core/src/services/search.ts	100% <0%> (ø)
...ges/@storefront/navigation/src/refinement/index.ts	100% <0%> (ø)
...s/@storefront/core/src/core/tag/mixins/metadata.ts	50% <0%> (ø)
packages/@storefront/core/src/services/redirect.ts	100% <0%> (ø)
... and 196 more

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