add checkpoints for mainnet blocks 300000 and 305000 and testnet block 250000
add checkpoints for mainnet blocks 300000 and 305000 and testnet block 250000

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