[vim] Use "system" fzf when available

1. Go binary: ../bin/fzf
2. System fzf: $(which fzf)
3. Download fzf from GitHub or create wrapper script to Ruby version (../fzf)
   when the binary for the platform is not available
4. If install script is not found or for some reason failed, try to use Ruby
   version in its expected location (../fzf)
5. If fzf is found to be a shell function, use it (type fzf)

plugin/fzf.vim

@@ -36,17 +36,17 @@ function! s:fzf_exec()
   if !exists('s:exec')
     if executable(s:fzf_go)
       let s:exec = s:fzf_go
-    elseif !s:installed && executable(s:install)
-      echohl WarningMsg
-      echo 'Downloading fzf binary. Please wait ...'
-      echohl None
-      let s:installed = 1
-      call system(s:install.' --bin')
-      return s:fzf_exec()
     else
       let path = split(system('which fzf 2> /dev/null'), '\n')
       if !v:shell_error && !empty(path)
         let s:exec = path[0]
+      elseif !s:installed && executable(s:install)
+        echohl WarningMsg
+        echo 'Downloading fzf binary. Please wait ...'
+        echohl None
+        let s:installed = 1
+        call system(s:install.' --bin')
+        return s:fzf_exec()
       elseif executable(s:fzf_rb)
         let s:exec = s:fzf_rb
       else