build: build-prod is taking too much time

[SuperITMan]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[X] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

When running npm run build:prod in showcase, it takes too much and slows down the tests on Travis.

Expected behavior

This should be faster to and should not slow down the tests on Travis

[christophercr]

As discussed with @SuperITMan, we will adapt the Travis config and the webpack prod config:

• we will run the uglification process just once and without sourcemaps for every PR. This will speed up the development process.
• we will run the uglification process 3 times and with sourcemaps for every release (on tag branches) and on nightly builds. In this way we trigger a full prod build at least once a day so we can be sure everything is working as expected.