Fix Range#{include?,member?} for X19

It should basically depend element.succ from X19.
And old behavior is keeping by Range#cover?.

Example
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    ('a'..'c').include? 'bc'

Results
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* MRI 1.8.7    #=> true
* MRI 1.9.3    #=> false
* Rubinius X18 #=> true
* Rubinius X19 #=> true  #Fix here