Let #exchange_to pass block to #exchange_with.

* Make Money#exchange_to pass a block to Money::Bank::*#exchange_with which then
  uses it for rounding.
* Add spec the yielding of the block.
Rerun specs on Travis.
Merge pull request #362 from ct-clearhaus/exchange_to_with_rounding_block

Let #exchange_to pass block to #exchange_with.