Test with newer versions of serf

[KushalP]

As of creation, the latest version of serf available is 0.8.1. We should test with this version to make sure that we can support it.

[boyski33]

Are you implying manual or automated testing?

[KushalP]

@boyski33 ideally, we would do this as a matrix build so that we can progressively add/remove versions of serf that we want to support. We would then use this parameter to drive this blog of the build: https://github.com/KushalP/serfclient-py/blob/09b3758/.travis.yml#L11-L15