Bump minimum OS X deployment version to 10.15

Signed-off-by: Darby Johnston <darbyjohnston@yahoo.com>

Author: darbyjohnston

CMakeLists.txt

@@ -16,7 +16,7 @@ set(OTIO_AUTHOR_EMAIL "otio-discussion@lists.aswf.io")
 set(OTIO_LICENSE      "Modified Apache 2.0 License")
 
 set(CMAKE_OSX_ARCHITECTURES "arm64;x86_64" CACHE STRING "")
-set(CMAKE_OSX_DEPLOYMENT_TARGET "10.14" CACHE STRING "Minimum OS X deployment version")
+set(CMAKE_OSX_DEPLOYMENT_TARGET "10.15" CACHE STRING "Minimum OS X deployment version")
 
 project(OpenTimelineIO VERSION ${OTIO_VERSION} LANGUAGES C CXX)