Less greedy algorithm for choosing builds

The previous algorithm had the problem that if you had builds

  A = browser+os.linux.x86_64
  B = browser+os.osx.x86_64

available and came in looking for "something that satisfies 'browser'
and 'os.osx.x86_64'", Meteor would happily say:

   "Neato-keen!  'A' satisfies 'browser'.  And 'B' satisfies
    'os.osx.x86_64'!  Hey, that's all I needed!  Let's download
     both 'A' and 'B'!  Hooray!"

when choosing just 'B' would have been good enough.