Add fallback for available balance to be compatible with node version 6

[soerenbf]

Changes

• Add fallback for AccountInfo.accountAvailableBalance

Checklist

• [x] My code follows the style of this project.
• [x] The code compiles without warnings.
• [x] I have performed a self-review of the changes.
• [x] I have documented my code, in particular the intent of the hard-to-understand areas.
• [x] (If necessary) I have updated the CHANGELOG.