Update the Gas Tracker to Sync with Etherscan.

[jayblitz]

On Metaschool Gas tracker, though the gas syncs every 15sec, the value is the same at Low - 20 GWEI, Medium- 20 and High - 22 GWEI ( https://metaschool.so/gastracker/ethereum )

While on Etherscan, GWEI is at 4.
Would be great to update it and keep it in sync with Etherscan ( https://etherscan.io/ )