Use env in shebang line

By using

    #!/usr/bin/env python

as the shebang line, `jcli` will be run with the local `python`
executable, regardless of where in the filesystem it resides. This also
enables you to support running `jcli` with a `virtualenv` configuration.
Merge pull request #1 from RWJMurphy/master

Use `env` in the shebang line