check block version for aux blocks properly for version warning

UpdateTip() was checking block versions against one of two
possible AuxPoW versions. Added consts for version checks and
IsAuxPowVersion(int nVersion) for use with UpdateTip()'s
block version check.
Merge pull request #616 from patricklodder/1.8-dev-auxpow-version

check block version for aux blocks properly for version warning