build: bump minimum supported to 10.15

Author: codebytere

binding.gyp

@@ -18,7 +18,7 @@
     ],
     'defines': [ 'NAPI_DISABLE_CPP_EXCEPTIONS' ],
     "xcode_settings": {
-      "MACOSX_DEPLOYMENT_TARGET": "10.13",
+      "MACOSX_DEPLOYMENT_TARGET": "10.15",
       "SYSTEM_VERSION_COMPAT": 1,
       "OTHER_CPLUSPLUSFLAGS": ["-std=c++14", "-stdlib=libc++"],
       "OTHER_LDFLAGS": [