fixed active company growth stats

- no longer use proxy query for active companies per month
- use same calculation as active displays (last average value of the period) for growth stats

trello card : https://trello.com/c/mDZZq7D8/56-fix-active-companies-growth-stats
relate PR: https://github.com/Rise-Vision/Dashboard-proxy/pull/38
PR hosted at : http://dashboard.risevision.com:8000

Could @ByronDarlison , @tejohnso, and/or @alexey-rise review?