* ethcore: propose hardfork block number 4230000 for ropsten
* ethcore: propose hardfork block number 9000000 for kovan
* ethcore: enable kip-4 and kip-6 on kovan
* etcore: bump kovan hardfork to block 9.2M
* ethcore: fix ropsten constantinople block number to 4.2M
* ethcore: disable difficulty_test_ropsten until ethereum/tests are updated upstream