tika: create integration tests for new server versions

[tbpg]

We should have a set of integration tests to make sure tika.Client works with new server versions. Came up in #8.

Could use an environment variable to indicate if the tests should run and t.Skip the test if not.

We know the list of supported versions (https://github.com/google/go-tika/blob/master/tika/server.go#L150), so the test can download the server, start it, then run whatever tests it needs.