Update Travis CI config.

PiperOrigin-RevId: 347391876

.travis.yml

@@ -4,7 +4,7 @@
 
 language: cpp
 dist: bionic
-osx_image: xcode11.3
+osx_image: xcode12.2
 
 compiler:
 - gcc
@@ -32,8 +32,8 @@ addons:
     packages:
     - clang-10
     - cmake
-    - gcc-9
-    - g++-9
+    - gcc-10
+    - g++-10
     - libgoogle-perftools-dev
     - libkyotocabinet-dev
     - libsnappy-dev
@@ -43,7 +43,7 @@ addons:
     packages:
     - cmake
     - crc32c
-    - gcc@9
+    - gcc@10
     - gperftools
     - kyoto-cabinet
     - llvm@10
@@ -59,7 +59,7 @@ install:
     export PATH="$(brew --prefix llvm)/bin:$PATH";
   fi
 # /usr/bin/gcc points to an older compiler on both Linux and macOS.
-- if [ "$CXX" = "g++" ]; then export CXX="g++-9" CC="gcc-9"; fi
+- if [ "$CXX" = "g++" ]; then export CXX="g++-10" CC="gcc-10"; fi
 # /usr/bin/clang points to an older compiler on both Linux and macOS.
 #
 # Homebrew's llvm package doesn't ship a versioned clang++ binary, so the values