Use github service containers in CI

[northernSage]

Following #35, using service containers as an alternative to manually installing chachelib external dependencies in tests.yml workflow (e.g. memcached, redis, pylibmc headers) seems like an interesting option.

Some thoughts:

• Two separate services will be necessary one for redis and one for memcached.
• The pytext-xprocess fixtures should not start local instances of redis and memcached when running under CI since the containers will be up. One way of going about this is checking one of the many environment variables set by the CI and having xprocess start (or not) based on the result.
• The github macOS does not have docker installed by default, so it will need to be set up.