Mark RubyLinter as abstract

Fixes #81

Author: kaste

linter.py

@@ -3,6 +3,8 @@
 
 
 class RubyLinter(Linter):
+    __abstract__ = True
+    
     def context_sensitive_executable_path(self, cmd):
         # The default implementation will look for a user defined `executable`
         # setting.