Fix tests that dig around in private stuff that changed in Twisted 14.0.0.
Remove debug print.
Revert "Remove debug print."

This reverts commit c4519abd170537579c2cec7d93b0cccf0c2a4524.
Revert "Fix tests that dig around in private stuff that changed in Twisted 14.0.0."

This reverts commit 73daa7787c9d2449bb539f4fb454efecbea7248a.
Merge branch 'develop' into feature/issue-801-fix-sandbox-tests-for-Twisted-14
Adjust Twisted version requirement and run Travis builds for oldest and newest versions.
Support Twisted 14.0.0 in sandbox HTTP client.
Add service_identity dependency to improve SSL hostname verification on newer Twisted versions.
No need to build against old Twisted in multiple Python versions.
Update explanatory comment in tests.
Another explanatory comment.
Merge branch 'feature/issue-801-fix-sandbox-tests-for-Twisted-14' into develop