Better organize integration tests

Add rake tasks `test:units` and `test:integration`

Run integration tests by default on TravisCi.
Run memcached and redis on TravisCi.
Integration tests: less flakiness by sleeping more
[travisci] Fix Errno::ENETUNREACH errors in redis integration tests
[travisci] Add ruby 2.1.1 tests
Update changelog
Merge branch 'integration'