Normalize all quote chars to double quotes on tests.
Remove commented test.
Add ttl method.
Add iter_keys method.
Remove duplicated imports (flake8 fixes)
Improve keys method.
Update changelog.
Update travis configuration.
Cosmetic fixes on redis_cache.cache (flakes8)
Cosmetic fixes on herd client (flakes8 fixes)
Cosmetic fixes on sharded client (flakes8 fixes)
Cosmetic fixes on pool module (flakes8 fixes)