Change ultisnips load condition

Benefit of doing this:

+ No change of cursor line position after entering insert mode. Previous,
  if we load ultisnips on InsertEnter, the cursor line will moved
  automatically (e.g., if cursor line is on bottom of the screen, when
  we enter insert mode, now we are at the middle of the screen, like
  Ctrl-E is used before entering insert mode), which is really annoying!
+ We can see the snippets in nvim-compe auto-completion menu. Previously,
  the snippets are not shown on the nvim-compe completion menu (snippet
  expansion works though, it is just we can not see snippets in the
  completion menu), possibly due to the loading order of ultisnips and
  nvim-compe, because we load them both on event InsertEnter. After
  changing ultisnips to a start plugin, this issue is gone.

lua/plugins.lua

@@ -126,7 +126,7 @@ require('packer').startup(
     end
 
     -- Snippet engine and snippet template
-    use {'SirVer/ultisnips', event = {'InsertEnter'}}
+    use 'SirVer/ultisnips'
     use {'honza/vim-snippets', event = {'InsertEnter'}}
 
     -- Automatic insertion and deletion of a pair of characters