ci: Fix travis-CI build matrix

Change-Id: Id44f23dba027559d8f96aca581840d325c5ca858
Refs: #3360
diff --git a/.travis.yml b/.travis.yml
index 9503148..6bf8602 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,12 +4,12 @@
   include:
     - os: linux
       dist: trusty
-      compiler: gcc
+      env: CXX=g++
     - os: linux
       dist: trusty
-      compiler: clang
+      env: CXX=clang++
     - os: osx
-      compiler: clang
+      env: CXX=clang++
 notifications:
   email:
     on_success: always