Remove 'Hold down "Control" ...' message from help_text

When getting the help_text from a field where `many=True`, Django
appends 'Hold down "Control", or "Command" on a Mac, to select more than
one.' to the help_text. This makes some sense in Django's ModelForms,
but no sense in the API.
Merge branch 'remove-hold-down-control-messages' of git://github.com/rouge8/django-rest-framework into rouge8-remove-hold-down-control-messages
Use 'force_text', not 'unicode', for compat across python version