Upgrade CI checks to Go 1.15

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     - name: Checkout
@@ -42,7 +42,7 @@ jobs:
       max-parallel: 4
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
-        go-version: [1.11, 1.12, 1.13, 1.14]
+        go-version: [1.14, 1.15]
     steps:
     - name: Set up Go
       uses: actions/setup-go@v1
@@ -76,7 +76,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     - name: Checkout