Add Clang 6.0 to the Travis CI matrix.

Change-Id: I1c4a242cbc4f085315122dab4ef5f6496d7d2009
Reviewed-on: https://code-review.googlesource.com/24650
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/.travis.yml b/.travis.yml
index b5a2a94..3eba558 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -121,6 +121,15 @@
             - clang-5.0
       env:
         - MATRIX_EVAL="CC=clang-5.0 CXX=clang++-5.0"
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - llvm-toolchain-trusty-6.0
+          packages:
+            - clang-6.0
+      env:
+        - MATRIX_EVAL="CC=clang-6.0 CXX=clang++-6.0"
 
 before_install:
   - eval "${MATRIX_EVAL}"