Add Clang 7 to the Travis CI matrix.

Change-Id: Id233cad7ddb02561998603eb3da28f7600073450
Reviewed-on: https://code-review.googlesource.com/33230
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/.travis.yml b/.travis.yml
index 456083c..674cda5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -143,6 +143,17 @@
             - clang-6.0
       env:
         - MATRIX_EVAL="CC=clang-6.0 CXX=clang++-6.0"
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'deb https://apt.llvm.org/trusty/ llvm-toolchain-trusty-7 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+          packages:
+            - clang-7
+      env:
+        - MATRIX_EVAL="CC=clang-7 CXX=clang++-7"
 
 before_install:
   - eval "${MATRIX_EVAL}"