Only suggest instead of requiring versions of BigBlueButton

[mconf-daileon]

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Author Name: Leonardo Daronco (Leonardo Daronco)
Original Redmine Issue: 1957, http://dev.mconf.org/redmine/issues/1957

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The gem currently requires the version of BigBlueButton to be one of the versions listed in it, see https://github.com/mconf/bigbluebutton_rails/blob/v2.0.0/app/models/bigbluebutton_server.rb#L49

Given the stability of the API, it would be better to just suggest these versions, but allow the user to use the gem with other versions as well. This would also help when newer versions of the API are released.

Also, the list of versions supported could be taken from bigbluebutton-api-ruby, see https://github.com/mconf/bigbluebutton-api-ruby/blob/v1.5.0/lib/bigbluebutton_api.rb#L71