Reinstate skipped cache cloning test

We don't need an actual instance, we want to know that all
Cache children prevent cloning.
Reinstate APC, Memcache and SQLite tests on travis

Adds travis and cache module configuration to run the unit tests
for these drivers - which are currently being skipped - on all
supported environments.
Silently skip the Wincache tests on travis to reduce build noise

It's unlikely travis will ever be able to test this class, because
it only runs on windows. The big block of skipped tests creates a
lot of noise and will mask skipped tests that we actually care
about.

Therefore, on travis we skip the test class definition to force
silent skipping of the tests.
Add temporary workaround for expected HHVM test failures on memcache

Per discussion in #52, expected failures on HHVM will be converted
to skipped tests until 3.4 is released and on Travis.
Add travis build badges to README

[skip ci]
Merge pull request #52 from acoulton/3.3/bug/reinstate-skipped-ci-tests

Reinstate and fix skipped cache tests for 3.3
Revert accidentally-commited @wip tags on File and APC tests

Introduced by 37e69151c