Add Clang 10 to the Travis CI matrix.

Change-Id: Id4753eaee5fe43a831e6610baffbfd53eb5d4c11
Reviewed-on: https://code-review.googlesource.com/c/re2/+/54331
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/.travis.yml b/.travis.yml
index d79541e..a3740ce 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -169,6 +169,18 @@
             - clang-9
       env:
         - MATRIX_EVAL="CC=clang-9 CXX=clang++-9"
+    - os: linux
+      dist: xenial
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-10 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+          packages:
+            - clang-10
+      env:
+        - MATRIX_EVAL="CC=clang-10 CXX=clang++-10"
 
     - os: osx
       osx_image: xcode7.3