Upgrade client dev runtime to Node 6 LTS

[jpellizzari]

Currently, running the webpack dev server and express proxy is eating up a ton of CPU cycles, even when the build isn't running. Upgrading to Node 6 appears to have fixed the problem. This change does break jest tests, however, because the jest-cli package uses an unsupported path argument:

jestjs/jest#1088

Upgradingjest-cli should fix this, but there were breaking changes which will need to be resolved.

Also Node 6.9.1 just went into LTS, so it should be supported until Spring 2018.
https://nodejs.org/en/blog/release/v6.9.0/

[jpellizzari]

Test builds:
time docker build -t node4-build . : real 16m12.900s& real 8m39.124s
time docker build -t node6-build .: real 4m13.992s
time docker build -t node4-build . (npm i --production): real 2m51.585s
time docker build -t node6-build . (npm i --production): real 1m43.083s
time docker build -t node6-yarn-build . (yarn --production): real 3m36.824s