Bumped docker build to use xenial
Bumped docker build to use xenial

The bump to tornado 5.x required newer version of python which wasn't avaialble on trusty and was causing build failures. CTR

apache/tinkerpop,krlohnes/tinkerpop,krlohnes/tinkerpop,robertdale/tinkerpop,robertdale/tinkerpop,apache/tinkerpop,apache/tinkerpop,apache/tinkerpop,krlohnes/tinkerpop,apache/tinkerpop,apache/incubator-tinkerpop,apache/tinkerpop,robertdale/tinkerpop,apache/incubator-tinkerpop,krlohnes/tinkerpop,krlohnes/tinkerpop,robertdale/tinkerpop,apache/incubator-tinkerpop,robertdale/tinkerpop,apache/tinkerpop