Upgraded G -> M from 164 [1654114758938]

[code423n4]

Judge has assessed an item in Issue #164 as Medium risk. The relevant finding follows:

File:Cally.sol#224
require(msg.value >= premium, "Incorrect ETH amount sent");
Using == operator is 3 gas cheaper than >=. This also keeps people from acedently overpaying when buying an option.

Summary
Implementing each of these would reduce the deploymnt cost from 5,431,682 to 5,387,621 and reduces buyOption from 75,765 to 75,013.

[HardlyDifficult]

Dupe of #84