versionCompatibility caching

[Turbo87]

In ember-test-selectors we've had issues using the versionCompatibility feature on TravisCI (see
mainmatter/ember-test-selectors#59 (comment)). It appeared that the Ember versions resolution was cached in some way. mainmatter/ember-test-selectors#60 did not fix it, but after deleting all caches on TravisCI everything suddenly worked as expected.

/cc @rwjblue @kategengler @marcoow

[Turbo87]

this seems to be related to ember-cli/ember-try-config#9 (comment)