Use gnus-other-frame for check mail key binding

b7671ff binds "C-c m" to "(gnus 1)". This behaves like I want when gnus
is not loaded (it starts up gnus and only fetches level 1 news), but
when gnus is already loaded, this command will cause all levels to be
fetched. Switching to `gnus-other-frame', which fetches level 1 in both
cases.