Make travis build succeed with node 12.13.0

[IreneStr]

In the .travis.yml, we used to specify the usage of the lts version of node. However, the Travis builds started failing when Travis started using node 12.13.0 instead of 10.16.3 as lts version. Therefore, in #382 we hardcoded the node version to 10.16.

However, this is not a longterm solution. 10.16 also has the LTS status until April 2020, but after that, 12.13 will be the LTS version.

These are my findings thus far:

• The minimum node-sass version for Node 12 is 4.12: https://github.com/sass/node-sass#supported-nodejs-versions-vary-by-release-please-consult-the-releases-page-below-is-a-quick-guide-for-minimum-support
• Upgrading node-sass to 4.12 doesn't solve the problem: f6b382a has a failing build.
• I also found some indications that we might need a npm rebuild of node-sass to make it work, but yarn doesn't have a rebuild command

NB. The same errors also appear locally if you upgrade to node 12.13.

Related issues:
Yoast/wordpress-seo#13691
https://github.com/Yoast/wordpress-seo-premium/issues/2591