Show that numbers larger than uint64 cannot be parsed

Closes #9

semver_test.go

@@ -131,6 +131,11 @@ var wrongformatTests = []wrongformatTest{
 	{nil, "-1.1.1"},
 	{nil, "1.-1.1"},
 	{nil, "1.1.-1"},
+	// giant numbers
+	{nil, "20000000000000000000.1.1"},
+	{nil, "1.20000000000000000000.1"},
+	{nil, "1.1.20000000000000000000"},
+	{nil, "1.1.1-20000000000000000000"},
 	// Leading zeroes
 	{nil, "01.1.1"},
 	{nil, "001.1.1"},