On Linux, build with GCC {4.9,5} and Clang 3.{6,7}

This gives us full C++11 support and avoids a problem with Catch
framework (which we're going to use for testing):
catchorg/Catch2#334

The recipe was lifted from
http://genbattle.bitbucket.org/blog/2016/01/17/c++-travis-ci/

.travis.yml

@@ -1,14 +1,56 @@
-os:
- - linux
- - osx
+language: cpp
+
+matrix:
+ include:
+ - compiler: gcc
+ os: linux
+ addons:
+ apt:
+ sources:
+ - ubuntu-toolchain-r-test
+ packages:
+ - g++-4.9
+ env: COMPILER=g++-4.9
+ - compiler: gcc
+ os: linux
+ addons:
+ apt:
+ sources:
+ - ubuntu-toolchain-r-test
+ packages:
+ - g++-5
+ env: COMPILER=g++-5
+ - compiler: clang
+ os: linux
+ addons:
+ apt:
+ sources:
+ - ubuntu-toolchain-r-test
+ - llvm-toolchain-precise-3.6
+ packages:
+ - clang-3.6
+ env: COMPILER=clang++-3.6
+ - compiler: clang
+ os: linux
+ addons:
+ apt:
+ sources:
+ - ubuntu-toolchain-r-test
+ - llvm-toolchain-precise-3.7
+ packages:
+ - clang-3.7
+ env: COMPILER=clang++-3.7
+ - compiler: gcc
+ os: osx
+ env: COMPILER=g++
+ - compiler: clang
+ os: osx
+ env: COMPILER=clang++
 
 install:
+ - export CXX=$COMPILER
  - cd ${TRAVIS_BUILD_DIR}
  - ./travis-get-deps.sh
-language: cpp
-compiler:
- - gcc
- - clang
 
 script:
  - cd ${TRAVIS_BUILD_DIR}

travis-get-deps.sh

@@ -5,9 +5,7 @@ case $TRAVIS_OS_NAME in
  sudo apt-add-repository -y ppa:ondrej/php5
  sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
  sudo apt-get update
- sudo apt-get install -qq libsqlite3-dev libcurl4-openssl-dev libxml2-dev libstfl-dev libjson-c-dev libncursesw5-dev bc gcc-4.9 g++-4.9
- sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.9 50
- sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.9 50
+ sudo apt-get install -qq libsqlite3-dev libcurl4-openssl-dev libxml2-dev libstfl-dev libjson-c-dev libncursesw5-dev bc
  ;;
 
  "osx")