Mark source as volatile (#844)

Deoplete expects to carry out the filtering of snippets by default, but the UltiSnips source uses UltiSnips#SnippetsInCurrentScope() which filters the returned snippets based on user input. Without is_volatile, Deoplete will cache the first set of results UltiSnips returns and keep trying to filter that, rather than getting a new set of results when the input changes.

rplugin/python3/deoplete/sources/ultisnips.py

@@ -7,6 +7,7 @@ class Source(Base):
         self.name = 'ultisnips'
         self.mark = '[US]'
         self.rank = 8
+        self.is_volatile = True
 
     def gather_candidates(self, context):
         suggestions = []