Fix typo in Heroku API provider
Fix typo in Heroku API provider

This is harmlss, since @source_blog was used for internal
memoization only.

jonathan-s/dpl,mathiasrw/dpl,Zyko0/dpl,Misfit-SW-China/dpl,computology/dpl,mattk42/dpl,travis-ci/dpl,travis-ci/dpl,pengsrc/dpl,clayreimann/dpl,travis-ci/dpl,testfairy/dpl,georgantasp/dpl,testfairy/dpl,testfairy/dpl