[AIRFLOW-1239] Fix unicode error for logs in base_task_runner
[AIRFLOW-1239] Fix unicode error for logs in base_task_runner

The details here are that there exists a PR for
this JIRA already (https://github.com/apache/incubator-
airflow/pull/2318). The issue is that in python 2.7 not
all literals are automatically unicode like they
are in python 3. That's what's the root cause, and
that can simply be fixed by just explicitly
stating all literals should be treated as unicode,
which is an import from the `__future__` module.
https://stackoverflow.com/questions/3235386/python-using-format-on-
a-unicode-escaped-string also explains this
same solution, which I found helpful.

Closes #2496 from Acehaidrey/master

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