advertising: Fix display of cpm in platform blurb.

Turns out calling 'min' on these doesn't work. I'm not really sure what it's doing.
Casting them to decimals first does the trick, though.  I'm 95% sure I checked this
before pushing it out, but maybe I'm just crazy.

reddit_about/pages.py

@@ -99,10 +99,10 @@ def __init__(self, *args, **kwargs):
         blurbs = SelfServeBlurb.get_all(return_dict=True)
         if 'platform' in blurbs:
             min_cpm = min([
-                g.cpm_selfserve_collection,
-                g.cpm_selfserve,
-                g.cpm_selfserve_geotarget_metro,
-            ]).decimal
+                g.cpm_selfserve_collection.decimal,
+                g.cpm_selfserve.decimal,
+                g.cpm_selfserve_geotarget_metro.decimal,
+            ])
             formatted_min_cpm = format_currency(min_cpm, 'USD', locale=c.locale)
             formatted_min_bid = format_currency(g.min_promote_bid, 'USD', locale=c.locale)
             blurbs['platform'].text = blurbs['platform'].text.replace(