pin less to less than 1.7.5

1.7.5 drop all vendor-prefixed values that are set to the default value
of most browser leading to huge change in the number of lines in css and
pain everywhere.

IPython/html/fabfile.py

@@ -12,7 +12,7 b" components_dir = pjoin(static_dir, 'components')"
 here = os.path.dirname(__file__)
 
 min_less_version = '1.7.0'
-max_less_version = '1.8.0' # exclusive
+max_less_version = '1.7.5' # exclusive
 
 def _need_css_update():
     """Does less need to run?"""