Use python2 as default.

In accordance with PEP-394, python2 scripts should use `python2` as an
interpreter. This ensures compatibility with setups that use python3 as
a default, while retaining compatibility with distributions that still
ship python2 as a default and comply with PEP-394.
Merge pull request #60 from hobarrera/develop

Use python2 as default. (rebased pull request #59)