update CI configuration

Author: glessard

.travis.yml

@@ -17,7 +17,7 @@ jobs:
       env: SWIFT=5.2.4
 
     - os: osx
-      osx_image: xcode12
+      osx_image: xcode12.1
       env: SWIFT=5.3
 
     - os: linux
@@ -29,6 +29,9 @@ jobs:
     - os: linux
       env: SWIFT=5.2.5
 
+    - os: linux
+      env: SWIFT=5.3
+
 before_install:
   - clang --version
   - . Utilities/install-swift.sh