Use getbufvar in an old-fashioned way for backward compatibility (#247)

getbufvar accepts only two arguments in Vim 7.3 (at least the one
included in OS X 10.11.5), and returns an empty string when the buffer
or variable does not exist: getbufvar({buffer}, {variable}).

Vim 7.4 has extended the built-in function so that it additionally
accepts an optional argument which determines the default return value:
getbufvar({buffer}, {variable}, [, {default}]). When the third argument
is not given and the buffer or variable does not exists, getbufvar
returns an empty string as before.

getbufvar({buffer}, {variable}) works in both Vim 7.3 and 7.4, and is
practically equivalent to getbufvar({buffer}, {variable}, 0) because an
empty string is coerced to 0 if necessary.

plugin/NERD_commenter.vim

@@ -515,7 +515,7 @@ function s:SetUpForNewFiletype(filetype, forceReset)
             endif
         endfor
         " if g:NERD_<filetype>_alt_style is defined, use the alternate style
-        let b:NERDCommenterFirstInit = getbufvar(1,"NERDCommenterFirstInit",0)
+        let b:NERDCommenterFirstInit = getbufvar(1,"NERDCommenterFirstInit")
         if exists('g:NERDAltDelims_'.ft) && eval('g:NERDAltDelims_'.ft) && !b:NERDCommenterFirstInit
             call s:SwitchToAlternativeDelimiters(0)
             let b:NERDCommenterFirstInit = 1