add go 1.15 to testing matrix and remove 1.13 (#68)

Author: JohnRoesler

.github/workflows/go_test.yml

@@ -12,8 +12,8 @@ jobs:
     strategy:
       matrix:
         go-version:
-          - 1.13
           - 1.14
+          - 1.15
     runs-on: ubuntu-latest
     steps:
       - name: Install Go