Enable PyPy tests on Travis

.travis.yml

@@ -17,10 +17,30 @@ matrix:
       env: TOXENV=py35
     - python: 3.6
       env: TOXENV=py36
+    - python: 2.7
+      env: TOXENV=pypy
     - python: 3.6
       env: TOXENV=docs
+  allow_failures:
+    - python: 2.7
+      env: TOXENV=pypy
 install:
- - pip install -U tox twine wheel codecov
+  - |
+      if [ "$TOXENV" = "pypy" ]; then
+        export PYENV_ROOT="$HOME/.pyenv"
+        if [ -f "$PYENV_ROOT/bin/pyenv" ]; then
+          pushd "$PYENV_ROOT" && git pull && popd
+        else
+          rm -rf "$PYENV_ROOT" && git clone --depth 1 https://github.com/yyuu/pyenv.git "$PYENV_ROOT"
+        fi
+        # get latest PyPy from pyenv directly (thanks to natural version sort option -V)
+        export PYPY_VERSION=`"$PYENV_ROOT/bin/pyenv" install --list |grep -o -E 'pypy-[0-9][\.0-9]*$' |sort -V |tail -1`
+        "$PYENV_ROOT/bin/pyenv" install --skip-existing "$PYPY_VERSION"
+        virtualenv --python="$PYENV_ROOT/versions/$PYPY_VERSION/bin/python" "$HOME/virtualenvs/$PYPY_VERSION"
+        source "$HOME/virtualenvs/$PYPY_VERSION/bin/activate"
+      fi
+  - pip install -U tox twine wheel codecov
+
 script: tox
 after_success:
   - codecov