Fix stripping of <bot> tag in reply_regexp

The <bot> tag was being passed through `utils.strip_nasties()` which was
removing all Unicode symbols even in UTF-8 mode. Instead, pass the bot
variables through `format_message()` to give them the same treatment as
normal messages.

This fixes #80 some more.

rivescript/brain.py

@@ -476,7 +476,7 @@ def reply_regexp(self, user, regexp):
         for var in bvars:
             rep = ''
             if var in self.master._var:
-                rep = utils.strip_nasties(self.master._var[var])
+                rep = self.format_message(self.master._var[var])
             regexp = regexp.replace('<bot {var}>'.format(var=var), rep)
 
         # Filter in user variables.