Always enable airline by setting laststatus=2

Previously the user was expected to set 'laststatus' himself to 2 if he
wanted to have airline be shown by default.

However it doesn't make much sense to have airline installed but not
display the statusline. Therefore, set the 'laststatus' to 2, if it
isn't set to it already.

plugin/airline.vim

@@ -106,6 +106,9 @@ function! s:airline_toggle()
             \ | call airline#load_theme()
     augroup END
 
+    if &laststatus < 2
+      set laststatus=2
+    endif
     if s:airline_initialized
       call s:on_window_changed()
     endif