Update Travis file to request GCC 4.9

The JSON library requires at least GCC 4.9

.travis.yml

@@ -5,6 +5,14 @@ env:
   global:
     - CMAKE_GENERATOR="Unix Makefiles" CMAKE_OPTIONS=" "
 
+addons:
+  apt:
+    sources:
+    - ubuntu-toolchain-r-test
+    packages:
+    - gcc-4.9
+    - g++-4.9
+
 matrix:
   include:
     - os: linux
@@ -18,6 +26,7 @@ matrix:
 install:
   - buildscripts/1-install-deps-travis.sh
   - export PATH="/data/bin:$PATH"
+  - if [ "$CXX" = "g++" ]; then export CXX="g++-4.9" CC="gcc-4.9"; fi
 
 script:
   - CMAKE_BUILD_TYPE=Debug buildscripts/2-build-travis.sh
@@ -26,5 +35,3 @@ script:
 after_success:
   - CMAKE_BUILD_TYPE=Debug buildscripts/3-release-travis.sh
   - CMAKE_BUILD_TYPE=Release buildscripts/3-release-travis.sh
-
-