allow to set an ethereum block number in Web3::Eth::Rpc

[donpdonp]

This patch sets a variable in Rpc to use a specific block number when calling the web3 service. It can be updated after the Rpc class is built. My use case is I want to look at a contract function return value for a specific block, then see how it has changed in the next block.

WEB3 = Web3::Eth::Rpc.new host: 'mainnet.infura.io', 
                          port: 443,  
                          block: 11453808

also replaces rails try with 'safe navigation' &.