Check against the right aux block version number

Previous commit 9e9babb was based on a wrong assumption, actual
CBlockHeader->nVersion for auxpow blocks is
CBlockHeader::CURRENT_VERSION | (AUXPOW_CHAIN_ID
   * BLOCK_VERSION_CHAIN_START)

and not BLOCK_VERSION_AUXPOW
Merge pull request #591 from patricklodder/1.8-version-check-fix

Check against the right aux block version number