Restrict the Travis CI Linux distro to Trusty.

matrix.include adds to the default builds which use Ubuntu Precise
which doesn't come with C++11 support by default.

Change-Id: I3c009bedc62aefd3363f749bb3cb22e9bc36fb9a
Reviewed-on: https://swiftshader-review.googlesource.com/11528
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/.travis.yml b/.travis.yml
index 8ba6ac0..d2a9c71 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,11 +1,11 @@
 language: cpp
 
-matrix:

-  include:

-    - os: linux

-      dist: trusty

-      sudo: false

-    - os: osx
+os:
+  - linux
+  - osx
+
+dist: trusty
+sudo: false
 
 compiler:
   - clang