Revert "Remove unused env value."

This reverts commit 3e7d928ddd.

This env value is needed to prevent run job with default compiler.

.travis.yml

@@ -16,7 +16,15 @@ branches:
     - master
     - develop
 
+env:
+  matrix:
+    - BOGUS_JOB=true
+
 matrix:
+
+  exclude:
+    - env: BOGUS_JOB=true
+
   include:
     - os: linux
       compiler: g++