Update Travis config

Author: sdispater

.travis.yml

@@ -51,12 +51,12 @@ install:
         source "$HOME/virtualenvs/pypy-$PYPY_VERSION/bin/activate"
       fi
   - |
-      if [ "$VENV" != "" ]; then
+      if [ "$VENV" != "" && "$VENV" != "pypy" ]; then
         virtualenv --python="$HOME/virtualenv/python$VENV/bin/python" "$HOME/virtualenvs/venv-$VENV"
         source "$HOME/virtualenvs/venv-$VENV/bin/activate"
       fi
-  - poetry install -v
-  - poetry build -v
+  - $HOME/virtualenv/python$TRAVIS_PYTHON_VERSION/bin/poetry install -v
+  - $HOME/virtualenv/python$TRAVIS_PYTHON_VERSION/bin/poetry build -v
   - pip --version
   - find dist/ -iname pendulum*.tar.gz -exec pip install {} \;