codec: .travis.yml: only test go 1.9+

testing.TB which we now leverage was introduced in go 1.9.

This gives us 4 releases (+ master) that we test on - should be sufficient.

.travis.yml

@@ -6,9 +6,9 @@ go:
   - 1.11.x
   - 1.10.x
   - 1.9.x
-  - 1.8.x
-  - 1.7.x
   # go testing suite support, which we use, was introduced in go 1.7
+  # testing.TB, TB.Helper() was introduced in go 1.9
+  # Consequently, tests will only compile for go 1.9+
 script:
   - go test -tags "alltests" -run Suite -coverprofile coverage.txt github.com/ugorji/go/codec
   - go test -tags "alltests safe" -run Suite -coverprofile coverage.safe.txt github.com/ugorji/go/codec