Add go1.18 to the test matrix

Github actions support go1.18 as well

.github/workflows/ci.yml

@@ -5,7 +5,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go: [ 1.17, 1.16, 1.15 ]
+        go: [ 1.18, 1.17, 1.16, 1.15 ]
     name: Tests Go ${{ matrix.go }}
     runs-on: ubuntu-18.04