Support for "q.op" parameter

[rkosenko]

Starting from Solr 7.0 this parameter is very essential as defaultOperator was dropped from the schema. Because of the dot in the name, it is not easy and straightforward to specify it in solrpy using kwargs. So, let's support it natively.

[rkosenko]

Looks like CI scripts are outdated:

Downloading archive: https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/16.04/x86_64/python-2.6.tar.bz2
0.10s$ curl -sSf --retry 5 -o python-2.6.tar.bz2 ${archive_url}
curl: (22) The requested URL returned error: 404 Not Found
Unable to download 2.6 archive. The archive may not exist. Please consider a different version.