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.