Fix the missing import for registering handlers in raven 2.X.

This bug was fixed in Raven 3.1.1, but the patch was never backported to
raven 2.x. This commit simply patch the commit 4431729220d45889144e29943d76d74badea7e9a
to the raven 2.0.12.2 code.
Merge pull request #536 from glasslion/2.0.12_bugfix

Bug fix for #529