Use callback directive for config_listener behaviour

This knocks out a few dialyzer errors such as:

`Callback info about the config_listener behaviour is not available`

It is also more descriptive as it specifies types and argument names for each
callback.
1 file changed
tree: 7706fa0fc9fc9e4392197b1093a5bba2c07f09f0
  1. .travis.yml
  2. LICENSE
  3. src/
  4. test/