Updates README to use 3.1.0
Updates README to use 3.1.0

mislav/will_paginate,poporul/will_paginate,CloudVLab/will_paginate,ThiagoLelis/will_paginate,notonthehighstreet/will_paginate,poporul/will_paginate,mislav/will_paginate,betterplace/will_paginate,CloudVLab/will_paginate,betterplace/will_paginate,notonthehighstreet/will_paginate,ThiagoLelis/will_paginate