- Fix Travis CI tests and updated dependencies

[IkoroVictor]

• HTTP 400 when interval param is set for plan update, ealthough documentation says its a valid param,https://paystack.com/docs/api/#plan-update. Removed param from tests
• Ignoring tests for /transferreciepts as it requires actual account validation and test payments accounts https://paystack.com/docs/payments/test-payments don’t work
• Use test accounts in subaccount tests
• Update hardcoded test account reference for transaction charge tests