Clear yarn cache on Travis (of incompatible sub-dependency)

[cyk]

This build is failing master and recent PRs (#652, #649, #648). This is caused by a sub-dependency specifying node compatibility >= 8:

error ip-regex@3.0.0: The engine "node" is incompatible with this module. Expected version ">=8". Got "6.16.0"
error Found incompatible module

The ip-regex is a sub-dependency of ember-cli-addon-docs. They ran into the same issue. The workaround was to pin the related dependency to an earlier version that supports Node 6.

Though ember-table's version constraint for ember-cli-addon-docs includes the fixed version, it appears that Travis's yarn cache continues to have the incompatible version (i.e., ip-regex@3.0.0).

TL;DR: Clearing the yarn cache on Travis should resolve the issue.

[Frozenfire92]

Is this best done via .travis.yml by disabling the cache?

ember-table/.travis.yml

Lines 14 to 15 in 821270b

	cache:
	yarn: true

or via Travis interface for each build?

https://docs.travis-ci.com/user/caching/#clearing-caches

Would love to get this going as #648 keeps popping up

[cyk]

@Frozenfire92 Good question. I think removing the yarn cache altogether is the better option. The reduced build time isn't worth (potentially) inconsistent builds.

[bantic]

Recent builds at Travis seem to be passing without much issue, so I will go ahead and close this, but if I'm mistaken please feel free to reopen.