go 1.10: update travis and gofmt (#1541)

* go 1.10: update travis and gofmt

fmt now complains about on file tls_test.go, fix that. Also remove
gofmt, as goimports also checks, so this was done twice.

Put go 1.9 and 1.10 in travis for the time being.

* goimports optional

plugin/tls/tls_test.go

@@ -18,8 +18,8 @@ func TestTLS(t *testing.T) {
 		expectedRoot       string // expected root, set to the controller. Empty for negative cases.
 		expectedErrContent string // substring from the expected error. Empty for positive cases.
 	}{
-	// positive
-	// negative
+		// positive
+		// negative
 	}
 
 	for i, test := range tests {