Avoid test failures on 32-bit archs MaxIPv4 is declared as an int, rather than a uint or a uint32, which leads to the following test failure on 32-bit archs (e.g. armel, armhf, i386, as seen on the awesome https://ci.debian.net/ platform):
iplib_test.go:362:30: cannot use MaxIPv4 (untyped int constant 4294967295) as int value in argument to t.Errorf (overflows)
Add an explicit cast to avoid the issue, until the upstream issue has been commented on.
