Update CI to use 11.7 instead of 11.6 (#281)

Author: mluisbrown

.github/workflows/ci.yml

@@ -15,8 +15,8 @@ jobs:
       matrix:
         xcode:
           - 11.4
-          - 11.5
-          - 11.6
+          - 11.7
+          - 12_beta
     steps:
       - uses: actions/checkout@v2
       - name: Select Xcode ${{ matrix.xcode }}