[ci] Remove gcc 4.9 in the Travis runs

We have been seeing intermittent failures with downloading the gcc-4.9
packages recently, so it's time to let this one go. gcc 4.9 hasn't been
updated for two years anyway, and Debian Stretch (released 2017-06-17)
includes gcc 6.3 as its default `gcc` package.

With all of this as the background information, I see very few
compelling reasons to retain gcc 4.9 support any more.

.travis.yml

@@ -3,17 +3,6 @@ dist: trusty
 
 matrix:
   include:
-    - os: linux
-      addons:
-        apt:
-          sources:
-            - ubuntu-toolchain-r-test
-          packages:
-            - gcc-4.9
-            - gcc-4.9-multilib
-      env:
-         - MATRIX_EVAL="CC=gcc-4.9 && ARCH=x86"
-
     - os: linux
       addons:
         apt: