enable travis-CI for the PyPy version

.travis.yml

@@ -26,6 +26,13 @@ matrix:
   - os: osx
     osx_image: xcode7.3
     env: PYTHON=3.5 CPP=14 CLANG
+  # Test a PyPy 2.7 nightly build
+  - os: linux
+    env: PYPY=1 PYTHON=2.7 CPP=11 GCC=4.8
+    addons:
+      apt:
+        sources: [ubuntu-toolchain-r-test, kubuntu-backports]
+        packages: [g++-4.8, cmake]
   # A barebones build makes sure everything still works without optional deps (numpy/scipy/eigen)
   # and also tests the automatic discovery functions in CMake (Python version, C++ standard).
   - os: linux
@@ -60,9 +67,18 @@ before_install:
   fi
   if [ -n "$CPP" ]; then export CPP=-std=c++$CPP; fi
   if [ "${PYTHON:0:1}" = "3" ]; then export PY=3; fi
+  if [ -n "$PYPY" ]; then
+    curl http://buildbot.pypy.org/nightly/trunk/pypy-c-jit-latest-linux64.tar.bz2 | tar -xj
+    export PYPY_BINARY=$(echo `pwd`/pypy-c-jit*/bin/pypy)
+    export CMAKE_EXTRA_ARGS="-DPYTHON_EXECUTABLE:FILEPATH=$PYPY_BINARY"
+  fi
   if [ -n "$DEBUG" ]; then export CMAKE_EXTRA_ARGS="-DCMAKE_BUILD_TYPE=Debug"; fi
 - |
-  # Initialize enviornment
+  # Initialize environment
+  if [ -n "$PYPY" ]; then
+    $PYPY_BINARY -m ensurepip
+    $PYPY_BINARY -m pip install pytest
+  fi
   if [ -n "$DOCKER" ]; then
     docker pull $DOCKER
     export containerid=$(docker run --detach --tty \