Use Go 1.15 (#277)

Update all CI checks and release process to use the latest patch version
of go1.15.

.github/workflows/gofmt.yml

@@ -14,10 +14,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.14
+    - name: Set up Go 1.15
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory