Upgrade pip to 1.3.1

This provides pip list --outdated that allows us to see which packages
are out of date.
Upgrade python packages

argh pymongo nose requests pep8 python-dateutil pytz
Remove creation of virtualenv

This forces the pip version back to default which in turn forces it to
be reinstalled when depdendencies are installed.
Create virtualenv on Jenkins only if doesn't exist

@robyoung
@pbadenski
Revert "Create virtualenv on Jenkins only if doesn't exist"

This reverts commit c9acc41409be4b9c3959cd78422860d91776c646.
Merge pull request #47 from alphagov/verify-packages

Update python packages to latest versions