truffle-config.js key update

Description

Made following changes in truffle-config.js:

1. For HDWalletProvider, replaced key with APIKEY
2. Changed the definition of DATAHUB_RPC_URL, used DATAHUB var in .env file

Fixes #21 (issue)

Type of change

Please mark the options that are relevant.

• Updated UI/UX
• Improved the business logic of code
• Added new feature
• Other

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings