Fix travis CI builds

[sobolevn]

After #3 was merged we have broken our CI.

Problem

Travis builds fail: https://travis-ci.org/sobolevn/jinja2-git/jobs/432398548
Traceback:

0.33s$ tox
ERROR: No setup.py file found. The expected location is:
  /home/travis/build/sobolevn/jinja2-git/setup.py
You can
  1. Create one:
     https://packaging.python.org/tutorials/distributing-packages/#setup-py
  2. Configure tox to avoid running sdist:
     https://tox.readthedocs.io/en/latest/example/general.html#avoiding-expensive-sdist
The command "tox" exited with 1.

Solution

We have to switch how our CI works. Currently it uses tox + setup.py which is gone.
But, we now need to use poetry instead.

Resources that might help you @mikhail-akimov :

1. poetry + tox travis example: https://github.com/sdispater/pendulum/blob/master/tox.ini
2. poetry travis example without tox: https://github.com/wemake-services/wemake-python-styleguide/blob/master/.travis.yml

Please, feel free to ask any questions you have.

[mikhail-akimov]

I think I fixed it with #5

[sobolevn]

Closed via #5