Travis CI: Use a newer version of Xcode

This may fix Tiled running on macOS Big Sur, since builds on GitHub Actions run
fine and they are made using this version of Xcode.

Also updated Qt to 5.12.10.

.travis.yml

@@ -7,7 +7,7 @@ env:
   global:
   - secure: J8Y+eQD0p5YPOlhBEPFgpZzhRFjXC4phiiFtGV3JaYX34wxCsyyNhMStMqIZTBL4D9lQojfLgXODBDaqT1iSHDe1oonosnyYnGBIJElez247pjuNiSSvcAbYa9RQnwfHe8JFphQHViURbv0OfqpHHyAlHy5b8nrPYWG2xmHA5nY=
   - secure: VgouZ5cStqlgODwIuLIvIBN5gkmrP95agCS3BorLm7g/iXM4cwuaV+bov9FiSlnzcnWfpsNMsFt5xDU2Oc17N0Kyx531MIA1H5h9xdzj/IhjhLcOOqYeXt3lN2quRE2maIZDSrPVr3eY0o6xdcNkeK0jpX/mgIiSHwXOclXv+6o=
-  - QT_VERSION=5.12.9
+  - QT_VERSION=5.12.10
   - QTCREATOR_VERSION=4.13.1
 addons:
   coverity_scan:
@@ -121,6 +121,7 @@ matrix:
     after_script:
     - cppcheck --enable=all -q -Isrc/libtiled `git ls-files src/\*.cpp`
   - os: osx
+    osx_image: xcode12
     addons:
       homebrew:
         packages: