update member_count/activitySummary calls to use new api

http://api.topcoder.com/v2/platform/statistics
update platform stat keys from old to new

active_members_count => activeMembersCount
prize_purse => prizePurse
active_contests_count => activeContestsCount
Merge pull request #136 from topcoderinc/dan-S-200115

Fix to update platform stats to use new API