Drop node 4 support

[TheDancingCode]

Closes #282.

[coveralls]

Coverage remained the same at 100.0% when pulling 5771dee on TheDancingCode:drop-node4 into 1945a05 on motdotla:master.

[maxbeatty]

Should remove from appveyor as well and add 10 to both CIs

[TheDancingCode]

Tests on Node 10 are currently failing due to an issue with lab leak detection. It is fixed in v15 of lab, but upgrading is not an option as they now require Node >= 8.9. I've opened an issue with them backport the fix to the v14 branch.

[maxbeatty]

This isn't the first time we've bumped up against issues with Lab. Might be time to switch to a test runner with more aligned support goals

[TheDancingCode]

Ava in turn breaks when stubbing fs and other internals, which is exactly what happens in these tests. What about Jest?

[maxbeatty]

Bummer. Ya we could try Jest or node-tap. Minimal setup and coverage reporting are really the two big features we're looking for

[TheDancingCode]

Don't really have time at the moment to rewrite the test suite though, so I'll leave that up to you.

[zenflow]

For now maybe consider just disabling the feature of Lab that's causing the problem? (leak detection) Is it needed? I think most test frameworks (like these alternatives) don't have that feature anyways. Still would be nice to use a more reliable test framework when someone has time to do that work.

[zenflow]

Still would be nice to use a more reliable test framework when someone has time to do that work.

Ah like node-tap in #304

[maxbeatty]

Closing this in favor of #304 would like to get Windows passing there then can cut a release