* core: s/isEIP155/isEIP2/ (fix)

This signature variable name reflects a spec'd change
in gas cost for creating contracts as documented in EIP2 (Homestead HF).

https://github.com/ethereum/EIPs/blob/master/EIPS/eip-2.md#specification

* core: s/isEIP2/sIsHomestead/g

Use isHomestead since Homestead is what the caller
and rest of the code uses.