Allow poetry's prereleases in Travis

Author: sdispater

.travis.yml

@@ -42,7 +42,7 @@ install:
       if [ "$TRAVIS_PYTHON_VERSION" != "3.6" ]; then
         virtualenv --python="$HOME/virtualenv/python3.6/bin/python" "$HOME/virtualenvs/venv-poetry"
       fi
-  - $HOME/virtualenvs/venv-poetry/bin/pip install poetry
+  - $HOME/virtualenvs/venv-poetry/bin/pip install poetry --pre
   - $HOME/virtualenvs/venv-poetry/bin/poetry install -v
   - $HOME/virtualenvs/venv-poetry/bin/poetry build -v
   - find dist/ -iname pendulum*.tar.gz -exec pip install {} \;