hotfix expect UnicodeDecode in _vim_enc

When running any snippets, it fail with message (below). 
The broken commit was 5ac0436694ae997b440a7a768d736f536ea59254 (first which produced this error).

Message:

An error occured. This is either a bug in UltiSnips or a bug in a              
snippet definition. If you think this is a bug, please report it to                                                                                                         
https://github.com/SirVer/ultisnips/issues/new.                                
                                                                               
Following is the full stack trace:                                             
Traceback (most recent call last):                                             
  File "/home/ivan/.vim/bundle/ultisnips/pythonx/UltiSnips/snippet_manager.py", line 54, in wrapper
    return func(self, *args, **kwds)                                            
  File "/home/ivan/.vim/bundle/ultisnips/pythonx/UltiSnips/snippet_manager.py", line 118, in expand
    if not self._try_expand():                                                  
  File "/home/ivan/.vim/bundle/ultisnips/pythonx/UltiSnips/snippet_manager.py", line 529, in _try_expand
    self._do_snippet(snippet, before)                                           
  File "/home/ivan/.vim/bundle/ultisnips/pythonx/UltiSnips/snippet_manager.py", line 474, in _do_snippet
    self._map_inner_keys()                                                      
  File "/home/ivan/.vim/bundle/ultisnips/pythonx/UltiSnips/snippet_manager.py", line 317, in _map_inner_keys
    " <C-R>=UltiSnips#JumpForwards()<cr>")                                      
  File "/home/ivan/.vim/bundle/ultisnips/pythonx/UltiSnips/_vim.py", line 88, in command
    return as_unicode(vim.command(as_vimencoding(cmd)))                        
  File "/home/ivan/.vim/bundle/ultisnips/pythonx/UltiSnips/compatibility.py", line 94, in as_vimencoding
    return _vim_enc(string)                                                    
  File "/home/ivan/.vim/bundle/ultisnips/pythonx/UltiSnips/compatibility.py", line 24, in _vim_enc
    return string.encode(vim.eval("&encoding"))                                
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 27: ordinal not in range(128)