Travis: Bump Ubuntu version to Xenial

- Update Ubuntu version to Xenial as Trusty is EOL as of April
- Drop GCC 4.9 build

.travis.yml

@@ -4,6 +4,8 @@ sudo: required
 
 cache: ccache
 
+dist: xenial
+
 matrix:
   include:
     - env: VERSION=8 BITS=64
@@ -15,9 +17,6 @@ matrix:
     - env: VERSION=7 BITS=32
       compiler: gcc
       os: linux
-    - env: VERSION=4.9 BITS=32
-      compiler: gcc
-      os: linux
     - env: VERSION=3.8 BITS=32
       compiler: clang
       os: linux