Removed docker build from travis.
Removed docker build from travis.

We can add this back if we like, but see DISCUSS thread: https://lists.apache.org/thread.html/rdc22accb40ed0c99f290a6846eb743488ce7c9e58c730c44fd9842ed%40%3Cdev.tinkerpop.apache.org%3E CTR

apache/tinkerpop,apache/tinkerpop,apache/incubator-tinkerpop,apache/incubator-tinkerpop,apache/tinkerpop,apache/tinkerpop,apache/incubator-tinkerpop,apache/tinkerpop,apache/tinkerpop,apache/tinkerpop