Fix bug 1022972 - strip leading "rv:" from version

Old Firefox clients may prefix their version with "rv:". This patch
strips that before performing a version compare.

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Review note: this is my first contribution to bedrock. I didn't test this patch. I didn't see any obvious locations for tests for this code. I would be happy to add a test.

Ping me on IRC if you need anything. I'm :gps.
I think if this were working correctly this test should fail. Looking at the code Fx 30.0 should get the tour if oldversion is < 29. I've checked out your branch and changed this test to the following and it works:

```python
    @override_settings(DEV=True)
    def test_rv_prefix(self, render_mock):
        """Prefixed oldversion shouldn't impact version sniffing."""
        req = self.rf.get('/en-US/firefox/whatsnew/?oldversion=rv:10.0')
        self.view(req, fx_version='30.0')
        template = render_mock.call_args[0][1]
        eq_(template, ['firefox/australis/whatsnew-tour.html'])
```

That said there is a test in this file that is specifically for the oldversion thing: 

https://github.com/mozilla/bedrock/blob/master/bedrock/firefox/tests/test_base.py#L757

Perhaps adding your test into that class would make the most sense.