Made ipdb optional.

requirements.txt

@@ -2,3 +2,4 @@ django<2
 python-dateutil
 beautifulsoup4
 translitcodec
+# ipdb

utils/templatetags/debug_utils.py

@@ -2,6 +2,7 @@
 from __future__ import unicode_literals
 # Erik Stein <code@classlibrary.net>, 2009-2017
 
+try:
     import ipdb
     from django import template
 
@@ -18,3 +19,6 @@ def ipdb_inspect(value):
     @register.simple_tag
     def ipdb_set_breakpoint():
         ipdb.set_trace()
+
+except:
+    pass