Have Travis CI run the Ajax unit tests

Fixes #93
Merge branch 'ajax-tests-travis'
Travis CI: don't run Ajax tests for 3.8 branch

There is a bug in versions prior to 3.9 that causes the super admin
privileges to leak to the wrong users. See [ticket
27205](https://core.trac.wordpress.org/ticket/27205) and [changeset
27706](https://core.trac.wordpress.org/changeset/27706).

See #93
Cache the points top users queries

Includes unit tests.

Fixes #89