Try to set ackprg on every invocation

[robin92]

Before this change, plugin would stop initialization unless g:ackprg has been found. This would prohibit from loading :Ack, :AckFromSearch and other autoload commands from being accessible. With this patch, plugin continues initialization when g:ackprg is missing and retries setting it when one of the autoload commands is called.

[robin92]

To give more context, I configure plugins using after directory. Setting g:ackprg there would cause :Ack command not to be present in VIM as g:ackprg would be set after the plugin has been initialized.