fixes bug 959601 - Graphics signature summary matview broken

@selenamarie r?

The culprit was [this commit](https://github.com/mozilla/socorro/commit/0b2a6810c6b4721ae0429ab1d4981138672a468d#diff-b9a3224d80ecf491d8e0e5bbb9612c67R265) .
It wasn't executing the [right stored procedure](https://github.com/mozilla/socorro/blob/master/socorro/external/postgresql/raw_sql/procs/update_signature_summary_graphics.sql)