ci: Use lowercase OS names to match API (#710)

* Change the 'matrix.os' parameter names to be all lowercase, following a change in the API

Author: matthewfeickert

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macOS-latest]
+        os: [ubuntu-latest, macos-latest]
         python-version: [2.7, 3.6, 3.7]
 
     steps:

.github/workflows/release_tests.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macOS-latest]
+        os: [ubuntu-latest, macos-latest]
         python-version: [3.7]
 
     steps: