Avoid OverflowError with large ranges

Use `IpRange.__len__()` directly rather than calling `len(IpRange())` when
computing the length of an IpRangeList. This won't stop OverflowErrors in
calling code such as `len(IpRangeList('fe80::/10'))`, but it will provide
a method to work around the overflow by direct use of `IpRangeList.__len__()`.

Issue: #12
Make doctest output portable

Python 3.x dropped the `L` suffix for the repr of long integers.
Remove Python 2.5 tests on Travis CI

As of 2013-11-20 Travis CI removed support for testing using python 2.5 See
http://blog.travis-ci.com/2013-11-18-upcoming-build-environment-updates/.
Merge pull request #13 from bd808/issue/12

Avoid OverflowError with large ranges