Fix tests and dependencies to support Tornado 5.0

[kevin-bates]

Issue #276 was introduced to limit the Tornado version to less than 5.0 since we're nearing a release boundary. This issue exists so that we can properly address Tornado 5.0 changes.

As mentioned in the referenced issue, Enterprise Gateway appears to operate against Tornado 5.0 once various modules within the jupyter stack have also been upgraded (namely, pyzmq, jupyter_client and notebook). However, failures still occur running the unit tests due to the removal of the previously deprecated class LogTrapTestCase.

At brief glance, I suspect changes will be required relative to ExpectLog (the recommended replacement for LogTrapTestCase) and in some ioloop handling. Notebook PR #3398 might prove helpful in this respect as might changes relative to Notebook Issue #3023.

Whatever we do here, we should look at applying to Kernel Gateway as well - since we inherited their tests.