Fix syntax error in #4515 on certain shells
Fix syntax error in #4515 on certain shells

Drops the `function` keyword and uses only `func(){}` syntax as per the
POSIX specification:

http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_09_05

Related: #4531

nunogt/oh-my-zsh,tekniklr/oh-my-zsh,nunogt/oh-my-zsh,tekniklr/oh-my-zsh