Adding ability to install zendopcache in CI.
Adding commented default build in CI.
Exclude incompatible builds xcache/APC on PHP 5.5.

Removing obsolete method.
Bump xcache version in CI.
Reenable xcache with 5.5.
Whitespace.
Adding ability to install APCu.
Correct docblocks in CI depends.
Run all tests in CI only when DB env variable is set.
Expect additional warning when connecting badly to MongoDb.