Replaced Bugsnag with Sentry - requires config changes!

Note that pillar/bugsnag_extra.py still exists; I'm keeping it around for
a while until we know what info we miss in Sentry, can port it, and then
remove/refactor it.

requirements.txt

@@ -5,7 +5,6 @@ attrs==16.2.0
 algoliasearch==1.12.0
 bcrypt==3.1.3
 blinker==1.4
-bugsnag[flask]==3.1.1
 bleach==1.4.3
 celery[redis]==4.0.2
 CommonMark==0.7.2
@@ -24,6 +23,7 @@ ndg-httpsclient==0.4.0
 Pillow==4.1.1
 python-dateutil==2.5.3
 rauth==0.7.3
+raven[flask]==6.3.0
 redis==2.10.5
 WebOb==1.5.0
 wheel==0.29.0