update from_currency to_currency pair in cache if
	last update for that pair is over 30 minutes ago by request API info

def update_cache(from_currency, to_currency):
	""" update from_currency to_currency pair in cache if
	last update for that pair is over 30 minutes ago by request API info """
	if check_update(from_currency, to_currency) is True:
		ccache[from_currency][to_currency]['value'] = convert_using_api(from_currency, to_currency)
		ccache[from_currency][to_currency]['last_update'] = time.time()
		cache.write(ccache)