Django 1.7 compatibility hotfix

Hi there,

This PR is more there to highlight an issue than to propose an actual fix, but:
- In Django 1.7, the paginator class no longer uses a range, but uses an "xrange" instead (on Python 2. On Python 3 xrange has become range).
- On Python 2, xranges aren't sliceable, so attempting to slice what we got returns in an error.

https://github.com/ericflo/django-pagination/blob/master/pagination/templatetags/pagination_tags.py#L144 

I suppose an actual fix should actually check for Python 2: there's no use coercing the range to a list on Python 3 as a Python 3 range can be sliced.