[WIP] Upgrade solidity to 0.4.23

[luisfidelis]

resolves #51

[luisfidelis]

• Upgrade Truffle to 4.1.7 (solc 0.4.23)
• Upgrade and rename zeppelin-solidity to [email protected]
• Use constructor() to define contract's constructor
• Add emit keyword when logging events
• Use view and pure instead of constant (will be deprecated)
• Add public visibility in some functions to avoid compilation warnings

[luisfidelis]

I need more details about those errors from paratii-js reported on #51

[jellegerbrandy]

Thank you very much, Luis, for this PR. It looks really good.

I'll comment on #51 on how to reproduce the errors.

[luisfidelis]

@jellegerbrandy i've commented on #51 on how to fix paratii-js tests errors regarding solidity 0.4.23/24. Tested with a reference to github:luisfidelis/paratii-contracts#aa8a3dd59cfe51f0d9dfff313d7b49c6fdcd8c43, the latest commit.