Change gitlab-ci to use yarn again, now using a later version of node.

2 jobs for webpack in 2 minutes and 1 second (queued for 3 seconds)
Name Stage Failure
build Build
npm ERR! whats-open-web@0.1.0 build: `webpack --config webpack/`
npm ERR! Exit status 2
npm ERR!
npm ERR! Failed at the whats-open-web@0.1.0 build script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! /root/.npm/_logs/2018-06-07T22_42_23_665Z-debug.log
ERROR: Job failed: exit code 1