CallbackMod never called

Due changes in commit 9c3122c9aba0e9908b437f3cee9856d14ea59778 setting
default return values for gettext_compile:get_env/1 the callback is
never called. It expects the function returns undefined.

Changed undefined to default values.
Merge pull request #16 from ncgmoviles/callbackmod_never_called

CallbackMod never called