Use pypy4 in travis tests

Author: syrusakbary

.travis.yml

@@ -7,6 +7,20 @@ python:
 - 3.5
 - pypy
 cache: pip
+before_install:
+  - |
+      if [ "$TRAVIS_PYTHON_VERSION" = "pypy" ]; then
+        export PYENV_ROOT="$HOME/.pyenv"
+        if [ -f "$PYENV_ROOT/bin/pyenv" ]; then
+          cd "$PYENV_ROOT" && git pull
+        else
+          rm -rf "$PYENV_ROOT" && git clone --depth 1 https://github.com/yyuu/pyenv.git "$PYENV_ROOT"
+        fi
+        export PYPY_VERSION="4.0.1"
+        "$PYENV_ROOT/bin/pyenv" install "pypy-$PYPY_VERSION"
+        virtualenv --python="$PYENV_ROOT/versions/pypy-$PYPY_VERSION/bin/python" "$HOME/virtualenvs/pypy-$PYPY_VERSION"
+        source "$HOME/virtualenvs/pypy-$PYPY_VERSION/bin/activate"
+      fi
 install:
 - pip install --cache-dir $HOME/.cache/pip pytest-cov pytest-mock coveralls flake8 isort==3.9.6 gevent==1.1b5 six>=1.10.0 promise>=0.4.0
 - pip install --cache-dir $HOME/.cache/pip pytest>=2.7.3 --upgrade