update solidity version

[jazzholicbae]

roll solidity version forward

closes https://github.com/SCBuergel/timeLock-smartContract/issues/1

ch

• [x] constant is no longer used.
• [x] set the visibility for each of the functions, made all public
• [x] Pay in is a function that sends money to the contract, so the function must be marked as payable.

Contract deployed on rinkeby

https://rinkeby.etherscan.io/address/0x718682935f5268e71b619e889c2064f7a730957e#code