There is one class in this project: Oystercard.
Oystercard contains the following methods:
* top_up
* deduct
*`balance`
*`in_journey`
*`touch_in`
*`touch_out`
touch_in will fail if balance is below the MINIMUM_FARE (1£)
touch_out will trigger deduction of MINIMUM_FARE
deduct method is passive and can therefore be made private.
Unit test involving deduct needs updating to reflect that.
Introduced a station argument to the touch_in method in order to enable journey tracking.
Within the Oystercard class, the instance variable entry_station was also introduced.
In order to run tests smoothly, pending do; end statements were used to isolate touch_in related tests.