Add g++-6 builds to travis-ci

The current linux/g++ testing (using a backported g++-4.8 on a
4-year-old Ubuntu) is quite ancient.  It's good as a baseline level of
support, but it means we aren't testing g++'s C++14 support at all
(which is why #334 happened).

This commit adds a docker-based travis-ci build using the debian
"testing" distribution, which will give us both another test system
(with different versions of build tools), while, more importantly, also
adding a build and test run using g++ in C++14 mode.

.travis.yml

@@ -4,20 +4,18 @@ matrix:
   include:
   - os: linux
     compiler: gcc-4.8
-    env: PYTHON=2.7 CPP=11
+    env: PYTHON=2.7 CPP=11 GCC=4.8
+    addons:
+      apt:
+        sources:
+        - ubuntu-toolchain-r-test
+        - kubuntu-backports # cmake 2.8.12
+        packages:
+        - g++-4.8
+        - cmake
   - os: linux
     compiler: gcc-4.8
-    env: PYTHON=3.5 CPP=11
-  - os: osx
-    osx_image: xcode7.3
-    env: PYTHON=2.7 CPP=14
-  - os: osx
-    osx_image: xcode7.3
-    env: PYTHON=3.5 CPP=14
-cache:
-  directories:
-  - $HOME/.cache/pip
-  - ccache
+    env: PYTHON=3.5 CPP=11 GCC=4.8
     addons:
       apt:
         sources:
@@ -26,13 +24,48 @@ addons:
         - kubuntu-backports # cmake 2.8.12
         packages:
         - g++-4.8
-      - python3.5
         - python3.5-dev
         - cmake
+  - os: osx
+    osx_image: xcode7.3
+    env: PYTHON=2.7 CPP=14
+  - os: osx
+    osx_image: xcode7.3
+    env: PYTHON=3.5 CPP=14
+  - compiler: gcc-6
+    services: docker
+    sudo: true
+    env: PYTHON=2.7 CPP=14 DOCKER=debian:testing NATIVE_DEPS=1
+    install:
+    - docker exec --tty "$containerid" apt-get -y --no-install-recommends install python2.7-dev python-scipy libeigen3-dev cmake make g++
+  - compiler: gcc-6
+    services: docker
+    sudo: true
+    env: PYTHON=3.5 CPP=14 DOCKER=debian:testing NATIVE_DEPS=1
+    install:
+    - docker exec --tty "$containerid" apt-get -y --no-install-recommends install python3.5-dev python3-scipy libeigen3-dev cmake make g++
+cache:
+  directories:
+  - $HOME/.cache/pip
+  - ccache
+before_install:
+- |
+  if [ -n "$DOCKER" ]; then
+    docker pull $DOCKER
+    export containerid=$(docker run --detach --tty \
+      --volume="$PWD":/pybind11 --workdir=/pybind11 \
+      --env="CC=$CC" --env="CXX=$CXX" --env="DEBIAN_FRONTEND=$DEBIAN_FRONTEND" \
+      --env=GCC_COLORS=\  \
+      $DOCKER)
+    docker exec --tty "$containerid" apt-get update
+    docker exec --tty "$containerid" apt-get -y upgrade
+    export SCRIPT_RUN_PREFIX="docker exec --tty $containerid"
+  fi
 install:
 - |
+  if [ -z "$NATIVE_DEPS" ]; then
     if [ "$TRAVIS_OS_NAME" = "linux" ]; then
-    export CXX=g++-4.8 CC=gcc-4.8;
+      if [ -n "$GCC" ]; then export CXX=g++-$GCC CC=gcc-$GCC; fi
       pip install --user --upgrade pip virtualenv
       virtualenv -p python$PYTHON venv
     elif [ "$TRAVIS_OS_NAME" = "osx" ]; then
@@ -47,10 +80,13 @@ install:
     fi
     source venv/bin/activate
     pip install numpy scipy
-- |
+
     wget -q -O eigen.tar.gz https://bitbucket.org/eigen/eigen/get/3.2.9.tar.gz
     tar xzf eigen.tar.gz
-  export CMAKE_INCLUDE_PATH=eigen-eigen-dc6cfdf9bcec
+    export CMAKE_EXTRA_ARGS="${CMAKE_EXTRA_ARGS} -DCMAKE_INCLUDE_PATH=eigen-eigen-dc6cfdf9bcec"
+  fi
 script:
-- cmake -DPYBIND11_PYTHON_VERSION=$PYTHON -DPYBIND11_CPP_STANDARD=-std=c++$CPP
-- CTEST_OUTPUT_ON_FAILURE=TRUE make check -j 2
+- $SCRIPT_RUN_PREFIX cmake ${CMAKE_EXTRA_ARGS} -DPYBIND11_PYTHON_VERSION=$PYTHON -DPYBIND11_CPP_STANDARD=-std=c++$CPP
+- $SCRIPT_RUN_PREFIX make CTEST_OUTPUT_ON_FAILURE=TRUE check -j 2
+after_script:
+- if [ -n "$DOCKER" ]; then docker stop "$containerid"; docker rm "$containerid"; fi