Update TravisCI to ignore false positive in btcec.

Also correct format verb found by go vet.

btcec/btcec_test.go

@@ -560,7 +560,7 @@ func TestBaseMultVerify(t *testing.T) {
 			data := make([]byte, bytes)
 			_, err := rand.Read(data)
 			if err != nil {
-				t.Errorf("failed to read random data for %s", i)
+				t.Errorf("failed to read random data for %d", i)
 				continue
 			}
 			x, y := s256.ScalarBaseMult(data)

goclean.sh

@@ -12,7 +12,7 @@ set -e
 # Automatic checks
 test -z $(gofmt -l -w . | tee /dev/stderr)
 test -z $(goimports -l -w . | tee /dev/stderr)
-test -z $(golint ./... | grep -v "ALL_CAPS" | grep -v "OP_" | tee /dev/stderr)
+test -z $(golint ./... | grep -v "ALL_CAPS" | grep -v "OP_" | grep -v "NewFieldVal" | tee /dev/stderr)
 go tool vet -structtags=false .
 env GORACE="halt_on_error=1" go test -v -race ./...