Increase block gas limit for parity instant seal dev chain

[cpurta]

Currently using the parity instant seal images for running tests against both locally and via travis CI. I am aware of contracts being quite large and requiring a lot of gas (8-10M). We are working on slimming down the contracts to eventually be deployed on the main net.

It would be good to still be able to deploy and test larger contracts without the need to rebuild a local image with the gas setting changed. I feel like a 1^7 gas limit would be sufficient for most anyones needs.