Fix duplicate role key in committee roster JSON

[lhotari]

Problem

The committee roster JSON endpoint fails with HTTP 500 for committees with a chair. Example: https://whimsy.apache.org/roster/committee/pulsar.json

sinatra.error = detected duplicate key "role" in {:name=>"Matteo Merli", :date=>"2018-09-19", :role=>"PMC member", "ldap"=>true, "githubUsername"=>"merlimat", "asf_githubStringID"=>"merlimat", :member=>true, "role"=>"PMC chair"}
sinatra.route = GET /committee/:name.json
REQUEST_URI = /roster/committee/pulsar.json

Cause

Roster entries store the role under the symbol key :role, but the chair's role was set with the string key 'role'. The chair's hash therefore has both :role and "role". Older versions of the json gem wrote "role" twice, while newer versions raise "detected duplicate key".

Fix

Set the chair role with :role in Committee (www/roster/models/committee.rb) and NonPMC (www/roster/models/nonpmc.rb), so that it replaces the existing value rather than adding a second key.