build: Compile with both clang and gcc on Travis

Fixes #1310

Change-Id: I430089a362b70e104ad3274a76b1033b2e94ac2a
diff --git a/.travis.yml b/.travis.yml
index d8b141c..36d70de 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,7 @@
 language: c++
+compiler:
+  - clang
+  - gcc
 os: linux
 dist: trusty
 before_install: sudo apt-get install git cmake build-essential bison libx11-dev libxcb1-dev libwayland-dev libmirclient-dev