Use Ethereum PPA for cryptopp and jsonrpc dependencies

@sveneh there is a difference between "Build from PPA" and "Install from PPA". `ppa:ethereum/ethereum-dev` provides the development snapshots for `cpp-ethereum` and `go-ethereum`, but we don't need those here. `ppa:ethereum/ethereum` provides the `libcryptopp` and `libjson-rpc-cpp` dependencies that we need.