drop ruby 1.9.3 travis support until contracthash_spec passes on it
drop ruby 1.9.3 travis support until contracthash_spec passes on it

bitpesa/bitcoin-ruby,assafshomer/bitcoin-ruby,Zlerp/bitcoin-ruby,robertg/bitcoin-ruby,tarix/bitcoin-ruby,georgemillo/bitcoin-ruby,xiumiao/bitcoin-ruby,mhanne/bitcoin-ruby