Override travis install method to use npm for older versions of node

[olingern]

I noticed that my PR was failing on node v4.xx.x on Travis CI, so I decided to look into it briefly.

Travis-CI introduced support for Yarn recently. Unfortunately, Yarn doesn't officially support node versions < v6.xx.x

Purposed fix
Override Travis install method to use npm install for node versions < v6.xx.x

[brendankenny]

from a failed travis run, it looks like the issue is one of our dependencies (devtools-timeline-model) specifying an engines entry (">=5.0") in its package.json and the fact that yarn enforces that by default.

It looks like there is a --ignore-engines flag for yarn which would help here, but we'd still have to rewrite the install section.

Another option is just doing

install:
  npm install

until we're ready to ditch older nodes (#449) and/or all-in on yarn.

[olingern]

I think the npm install option is good as it essentially reverts to the previous Travis-CI install method. It's unfortunate to not be able to take advantage of Yarn's caching, but --ignore-engines could potentially introduce more headaches from other dependencies.

[brendankenny]

awesome, thanks for catching this so quickly!