Updates from stevenmichaelthomas fork

[stevenmichaelthomas]

This brings all updates contained in my fork into master.

Updates include:

• Update ruby version
• Fix tests based on ruby version update
• Withdrawal typo fix
• Add dev notes and console rake task
• Add missing json and time requires
• Add figaro for env var management and testing
• Fix Bittrex::Wallet.all method
• Update readme with dev info

@mwerner

[mwerner]

Looks great! Thanks for the contribution