warrior.py: Fix Python 3 detecting missing task

Replaces `'foo' in e` with `'foo' in str(e)`.

Fixes ralphbean#127

Author: jayvdb

taskw/warrior.py

@@ -554,7 +554,7 @@ def get_version(cls):
                 stdout=subprocess.PIPE
             ).communicate()[0]
         except OSError as e:
-            if 'No such file or directory' in e:
+            if 'No such file or directory' in str(e):
                 raise OSError("Unable to find the 'task' command-line tool.")
             raise
         return LooseVersion(taskwarrior_version.decode())