improve caching performance in CI

[boneskull]

This change "warms up" the npm caches for Node.js 9 jobs, since it will be reused during the build. It re-enables caching for Node.js 8, 6 and 4, since I RTFM'd more closely. 😬

From the Travis-CI docs:

There is one cache per branch and language version/ compiler version/ JDK version/ Gemfile location/ etc.

1. for the smoke tests, we do a production only install per node.js version. we do NOT want to cache node_modules or ~/.npm, since it should always reflect what a user would get.
2. we warm up the cache; basically this runs a full npm install for node.js 9 with no global environment variables
3. we lint, using the warmed-up cache
4. tests:

• we run node.js tests in 9 and browser tests using the warmed-up cache
• we run node.js tests in v6 and v4 using a node_modules-only cache. these are not reused during the build (unlike the node 9 caches), but may be reused during subsequent builds against the same branch.

[coveralls]

Coverage remained the same at 89.955% when pulling b9d0397 on precache into ff9f3cf on master.

[boneskull]

I'm going to go ahead and merge this because it seems to work. Mainly just wanted to explain myself.