Fixed docs generation

[maxsam4]

• Fixed OpenZeppelin solidity-docgen (Increased max buffer size given to solc OpenZeppelin/solidity-docgen#17)
• Modified our script to fetch solc 0.4.24 instead of latest (0.5.0)
• Modified Travis script to fetch required dependencies
• Split the docs generation into a separate optional job that builds won't fail even if docs fail for some reason