Fix getBlockByHash tsc error

[cgewecke]

Tiny fix to get tests and coverage running as a baseline for new PRs (#151, #152, #153) coming in....

The most recent ethereumjs-util update had stricter types and latest builds here were failing with:

Type 'false' is not assignable to type 'string | number | number[] | Buffer | BN | Uint8Array | TransformableToArray | TransformableToBuffer | null | undefined'.

89       let block = await this._chain.getBlock(toBuffer(blockHash))
                                                         ~~~~~~~~~

Proposed change corrects the types so they match the docs description of the params arg:

An array of two parameters: A block hash as the first argument and a boolean determining 
whether it returns full transaction objects or just the transaction hashes

(This project .gitignore's package-lock.json)