Update main.h
Fix the PoW block number limit

Approximated calculation of the total number of PoW block to match the 100,000,000 PoW coins anounced in thread https://bitcointalk.org/index.php?topic=578963.0
Exact calculation should be made by the dev
Merge pull request #1 from hg5fm/master

Suggested fix for the maximum number PoW blocks problem