Update Travis CI to use Xcode 12.2 to support macOS 11.0 builds.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 503dc99..81a3a5d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,7 @@
 
 # macOS and Xcode Version
 os: osx
-osx_image: xcode12
+osx_image: xcode12.2
 
 # Build with verbose logging to avoid Travis timing out.
 script: