Travis: use pip 7 for automatic building of wheels
Travis: use pip 7 for automatic building of wheels

This simplifies the generating and caching of wheels, and makes it less
fragile (recently failed in
https://travis-ci.org/yourlabs/django-autocomplete-light/builds/63843767).

Tested in https://travis-ci.org/yourlabs/django-autocomplete-light/builds/63993857.

dsanders11/django-autocomplete-light,Visgean/django-autocomplete-light,Eraldo/django-autocomplete-light,Visgean/django-autocomplete-light,shubhamdipt/django-autocomplete-light,Perkville/django-autocomplete-light,yourlabs/django-autocomplete-light,yourlabs/django-autocomplete-light,dsanders11/django-autocomplete-light,shubhamdipt/django-autocomplete-light,Perkville/django-autocomplete-light,Eraldo/django-autocomplete-light,luzfcb/django-autocomplete-light,dsanders11/django-autocomplete-light,Eraldo/django-autocomplete-light,luzfcb/django-autocomplete-light,luzfcb/django-autocomplete-light,Visgean/django-autocomplete-light,Perkville/django-autocomplete-light,shubhamdipt/django-autocomplete-light,dsanders11/django-autocomplete-light,Perkville/django-autocomplete-light,Eraldo/django-autocomplete-light,yourlabs/django-autocomplete-light,shubhamdipt/django-autocomplete-light,luzfcb/django-autocomplete-light,Visgean/django-autocomplete-light,yourlabs/django-autocomplete-light