drop gcc-4.9 builds from Travis CI

Summary: gcc-4.9 is old and is no longer supported by folly.

Reviewed By: phoad

Differential Revision: D10408986

fbshipit-source-id: 68aa978f4f27ac1419aac26377433e399243ad37

Author: simpkins

.travis.yml

@@ -52,15 +52,6 @@ matrix:
             - clang-4.0
             - libstdc++-4.9-dev
 
-    - env: COMPILER=gcc-4.9
-      addons:
-        apt:
-          sources:
-            - *common_srcs
-          packages:
-            - *common_deps
-            - g++-4.9
-
     - env: COMPILER=gcc-5
       addons:
         apt: