Update CI workflow to test with Swift 5.3 (#2)

* Test with Swift 5.3

* Update package manifest to swift-tools-version 5.3

* Remove release configuration flag from swift test invocation

.github/workflows/ci.yml

@@ -10,20 +10,31 @@ jobs:
   macos:
     runs-on: macos-latest
 
+    strategy:
+      matrix:
+        xcode:
+          - "12" # Swift 5.3
+
+    name: "macOS (Xcode ${{ matrix.xcode }})"
+
     steps:
       - name: Checkout
         uses: actions/checkout@v1
       - name: Install libgit2
         run: brew install libgit2
       - name: Build and Test
         run: swift test
+        env:
+          DEVELOPER_DIR: /Applications/Xcode_${{ matrix.xcode }}.app/Contents/Developer
 
   linux:
     runs-on: ubuntu-latest
 
     strategy:
       matrix:
-        swift: ["5.2"]
+        swift: ["5.3"]
+
+    name: "Linux (Swift ${{ matrix.swift }})"
 
     container:
       image: swift:${{ matrix.swift }}

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.2
+// swift-tools-version:5.3
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription