Add stem CI for --target ONLINE

From https://trac.torproject.org/projects/tor/ticket/28170#comment:14

    Run 'run_tests.py --all' with python 2.6.
    Run 'run_tests.py --all' with python 2.7.
    Run 'run_tests.py --all' with python 3.x.
    Run 'run_tests.py --all --target ONLINE' with python 2.7.

Here's what I need to do:

check to see if Travis supports python 2.6, and add a job with 2.6
make the 2.7 job --target ONLINE
- assuming that ONLINE is a superset of offline
make the latest python 3.x job --target ONLINE
- assuming that ONLINE is a superset of offline

I'm modifying existing jobs to save build time: we could create separate jobs for ONLINE, but that would add ~5 minutes to every CI run.

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