Display calling line triggering deprecation warning

When logging a deprecation warning, it is really useful to display the calling line to open and change immediately.

For example, within the spec run:

```
DEPRECATION WARNING: as of Money 6.1.0 you must `require 'money/core_extensions'` to use Numeric#to_money. (called from: /Users/tom/src/current/money/lib/money/money/arithmetic.rb:29)
```