[build] Deprecated GCC 4.9 and set the minimum to GCC 5

circle.yml

@@ -78,12 +78,12 @@ workflows:
           metrics: true
           style_tests: true
       - build-template:
-          name: linux-gcc4.9-release
+          name: linux-gcc5-release
           executor_name: ubuntu-disco
           target_is_linux: true
           requires:
             - linux-gcc8-release
-          config_params: '-G Ninja -DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DCMAKE_C_COMPILER=gcc-4.9 -DCMAKE_CXX_COMPILER=g++-4.9'
+          config_params: '-G Ninja -DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DCMAKE_C_COMPILER=gcc-5 -DCMAKE_CXX_COMPILER=g++-5'
           build_params: '--target mbgl-glfw'
           test_params: '-N -Q'
       - build-template:

scripts/ci/Dockerfile

@@ -72,9 +72,7 @@ RUN set -eu \
     && apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1E9377A2BA9EF27F \
     && add-apt-repository "deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu xenial main" \
     && add-apt-repository "deb http://us.archive.ubuntu.com/ubuntu/ xenial main universe" \
-    && apt-get install -y \
-        g++-4.9 \
-        g++-5
+    && apt-get install -y g++-5
 
 # Install Android NDK
 RUN set -eu \