migrations/2_deploy_contracts.js: Increase gas limit.
migrations/2_deploy_contracts.js: Increase gas limit.

The gas increase is required to enable coverage, otherwise run out gas is
thrown. Instead of hardcoding the limit, take it from the current network.
On normal dev network the limit is much lower than on the coverage network.

Issue: #18

MarketProject/MarketProtocol,MarketProject/MarketProtocol