.travis.yml: enable gcc-5

.travis.yml

@@ -6,6 +6,14 @@ language: cpp
 compiler:
   - gcc
 
+addons:
+  apt:
+    sources:
+    - ubuntu-toolchain-r-test
+    packages:
+    - gcc-5
+    - g++-5
+
 before_install:
   - sudo apt-get install -y libgl1-mesa-dev libpulse-dev libxrandr-dev libgtk-3-dev