Don't attempt Python 3.7 on Travis CI yet
Don't attempt Python 3.7 on Travis CI yet

Travis CI doesn't support Python 3.7 yet (though it was released in
June 2018).

Change-Id: I1b0305f9c5d0b4dcafb762842f42b70fe6526274
Reviewed-on: https://gerrit.franz.com:9080/12353
Reviewed-by: Tadeusz Sznuk <4402abb98f9559cbfb6d73029f928227b498069b@franz.com>
Reviewed-by: Ahmon Dancy <8f7d8ce2c6797410ae95fecd4c30801ee9f760ac@franz.com>
Tested-by: Ahmon Dancy <8f7d8ce2c6797410ae95fecd4c30801ee9f760ac@franz.com>

franzinc/agraph-python,franzinc/agraph-python,franzinc/agraph-python,franzinc/agraph-python