Don't compare modified time if output file doesn't exists.

Fix for #329.

pipeline/compilers/__init__.py

@@ -81,6 +81,8 @@ class CompilerBase(object):
         return content
 
     def is_outdated(self, infile, outfile):
+        if not self.storage.exists(outfile):
+            return True
         try:
             return self.storage.modified_time(infile) > self.storage.modified_time(outfile)
         except (OSError, NotImplementedError):