CLI: upgrade cross-spawn to 6.0.3

[Hypnosphi]

Issue: #2815

The only breaking change is dropping support for node < 4, which we don't seem to support anyway

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This change is 

[codecov]

Codecov Report

Merging #2854 into master will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #2854      +/-   ##
=========================================
- Coverage   37.48%   37.3%   -0.18%     
=========================================
  Files         434     435       +1     
  Lines        9389    9432      +43     
  Branches      894     883      -11     
=========================================
  Hits         3519    3519              
- Misses       5318    5379      +61     
+ Partials      552     534      -18

Impacted Files	Coverage Δ
app/polymer/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/vue/src/server/utils.js	0% <0%> (-100%)	⬇️
app/polymer/src/server/babel_config.js	0% <0%> (-76.67%)	⬇️
lib/core/src/client/manager/preview.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/index.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/index.js	29.28% <0%> (ø)	⬆️
lib/core/src/client/preview/reducer.js	37.5% <0%> (ø)	⬆️
...ddons/actions/src/components/ActionLogger/index.js	74.35% <0%> (ø)	⬆️
lib/core/src/server/build-dev.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/routes.js	0% <0%> (ø)	⬆️
... and 65 more

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