get tidy5 binary always

Check path of tidy5 binary with Linter classmethod `which`:
http://www.sublimelinter.com/en/latest/linter_methods.html?highlight=executable_path#which

Fixes #31

Author: therealmarv

linter.py

@@ -11,15 +11,14 @@
 """This module exports the HtmlTidy plugin class."""
 
 from SublimeLinter.lint import Linter, util
-import shutil
 
 
 class HtmlTidy(Linter):
 
     """Provides an interface to tidy."""
 
     syntax = 'html'
-    if shutil.which('tidy5'):
+    if Linter.which('tidy5'):
         cmd = 'tidy5 -errors -quiet -utf8'
     else:
         cmd = 'tidy -errors -quiet -utf8'