Fix vim-capslock functionality after changes in CapsLockStatusline()

CapsLockStatusline() used to return '[caps]' when soft-capslock was
on... and now returns '[Caps]'.  As we're just using this to test for
the lock, we lower-case the return value in order to "normalize" the
value across versions.

The change in behaviour can be seen at:

tpope/vim-capslock@3a0f051373

autoload/airline/extensions/capslock.vim

@@ -5,7 +5,7 @@ if !exists('*CapsLockStatusline')
 endif
 
 function! airline#extensions#capslock#status()
-  return CapsLockStatusline() == '[caps]' ? 'CAPS' : ''
+  return tolower(CapsLockStatusline()) == '[caps]' ? 'CAPS' : ''
 endfunction
 
 function! airline#extensions#capslock#init(ext)