Enable Greenkeeper

[voxsim]

This issue is to enable Greenkeeper (https://greenkeeper.io/). The yarn project has multiple dependencies and this can help us to have everything update it.

We already tried #3375 but the main problems were:

• Greenkeeper didn't support yarn.lock file (Support yarn? greenkeeperio/greenkeeper#314). Now this is fixed thanks to greenkeeperio/greenkeeper-lockfile.
• Greenkeeper tries to update everything at the same time, before enabling it maybe we need to upgrade:
  [ ] babel-core to 6.25.0
  [ ] babel-loader to 7.1.1
  [ ] babel-preset-node5 to 12.0.1
  [ ] babylon to 6.17.4
  [ ] eslint to 4.2.0
  [ ] eslint-plugin-flowtype to 2.34.1
  [ ] eslint-plugin-jasmine to 2.7.1
  [ ] eslint-plugin-react to 7.1.0
  [ ] webpack to 3.1.0
  [ ] yargs to 8.0.2
  [ ] chalk to 2.0.1
  [ ] commander to 2.11.0
  [ ] gunzip-maybe to 1.4.1
  [ ] inquirer to 3.2.0
  [ ] micromatch to 3.0.3
  [ ] node-emoji to 1.7.0
  [ ] tar-fs to 1.15.3
  [ ] uuid to 3.1.0

This is an extract from yarn upgrade-interactive maybe something is wrong or maybe we can find a way to block the version on Greenkeeper.