Are you offering iOS development as a regular service? Then update costs for your tools should be factored into your rate. There is nothing wrong with having different rates for different work, so if the tools are especially pricy or you don´t sell so much hours just up your rate (or try to find more work in that area)
Do you only do this upon your clients request: They should provide the specialized tools. Normally this means they will provide you one of their devices and you have to give it back at the end of your assignment.
As usual, there is some grey area in between and if your client is happy to contribute a certain amount for you, updating your iPhone (as it means he does not have to provide one of theirs) you may have a win-win.