Update set of compilers run on Travis

Removing gcc 4.8 and 4.9, adding gcc 5.3.

First step towards #26216.

.travis.yml

@@ -36,7 +36,7 @@ jobs:
   include:
     # Initial test stage, if this fails everything else is cancelled.
     - stage: test
-      # GCC 5 is the pre-installed compiler on Xenial
+      # GCC 5.4 is the pre-installed compiler on Xenial
       env: COMPILER=g++ MODS=--mods=RL_Classes TEST_STAGE=1
 
     # Then build different configurations and targets in parallel.
@@ -91,21 +91,12 @@ jobs:
           packages: ["clang-8", "libc6-dbg", "libc6-dbg:i386", "g++-6", "libsdl2-dev", "libsdl2-ttf-dev", "libsdl2-image-dev", "libsdl2-mixer-dev"]
           sources: [*apt_sources, llvm-toolchain-trusty-8]
 
-    - env: COMPILER=g++-4.8
+    - env: COMPILER=g++
       compiler: gcc
-      addons: &gcc48
+      addons: &gcc53
         apt:
-          packages: ["g++-4.8", "g++-4.8-multilib", "libc6-dbg", "libc6-dbg:i386"]
-          sources: *apt_sources
-
-    # GCC 4.9
-    - env: COMPILER=g++-4.9
-      if: type != pull_request
-      compiler: gcc
-      addons: &gcc49
-        apt:
-          packages: ["g++-4.9", "g++-4.9-multilib", "libc6-dbg", "libc6-dbg:i386"]
-          sources: *apt_sources
+          packages: ["g++-5=5.3.1-14ubuntu2", "libstdc++-5-dev=5.3.1-14ubuntu2", "gcc-5=5.3.1-14ubuntu2", "gcc-5-base=5.3.1-14ubuntu2", "cpp-5=5.3.1-14ubuntu2", "libgcc-5-dev=5.3.1-14ubuntu2", "libasan2=5.3.1-14ubuntu2", "libmpx0=5.3.1-14ubuntu2"]
+          sources: [*apt_sources]
 
     # GCC 6
     - env: COMPILER=g++-6