Remove rbx from allowed failures section in travis config
Update travis configuration to match DM2 gems more closely
Fix Gemfile to reference https URL for rubygems
Fix Property#typecast to pass-through values that cannot be handled

* This fixes a related problem to #242. Most of properties that ship
  with dm-core define #typecast_to_primitive, but custom properties
  may not and I believe we should pass-through the value when it
  cannot be handled.