Use connection_pool for Travis::Features.
Add a Redis connection pool.
Revert "Use connection_pool for Travis::Features."

This reverts commit 5d4bc98545d838f32de6950db39d1c512278f6f3.
We use a direct pool proxy instead, so the original code can remain
unchanged.
Use the RedisPool instead of the Redis client.
Add tests for RedisPool.
Merge branch 'master' into mm-connection-pool-features
Merge branch 'master' into mm-connection-pool-features
Merge branch 'master' into mm-connection-pool-features
Merge pull request #214 from travis-ci/mm-connection-pool-features

Add a transparent connection pool for Redis