pin lessc to 1.4

Not necessarily what we want to do,
but every minor revision of less seems to change how it minifies,
which produces churn.

At least this should prevent accidental churn,
as has recently been committed to master.

IPython/html/fabfile.py

@@ -3,11 +3,15 b''
 from fabric.api import local,lcd
 from fabric.utils import abort
 import os
+from distutils.version import LooseVersion as V
+from subprocess import check_output
 
 pjoin = os.path.join
 static_dir = 'static'
 components_dir = os.path.join(static_dir, 'components')
 
+min_less_version = '1.4.0'
+max_less_version = '1.5.0' # exclusive
 
 def css(minify=True, verbose=False):
     """generate the css from less files"""
@@ -27,6 +31,16 b' def _compile_less(source, target, minify=True, verbose=False):'
     verbose = _to_bool(verbose)
     min_flag = '-x' if minify is True else ''
     ver_flag = '--verbose' if verbose is True else ''
+    
+    # pin less to 1.4
+    out = check_output(['lessc', '--version'])
+    out = out.decode('utf8', 'replace')
+    less_version = out.split()[1]
+    if V(less_version) < V(min_less_version):
+        raise ValueError("lessc too old: %s < %s" % (less_version, min_less_version))
+    if V(less_version) > V(max_less_version):
+        raise ValueError("lessc too new: %s > %s" % (less_version, max_less_version))
+    
     with lcd(static_dir):
         local('lessc {min_flag} {ver_flag} {source} {target}'.format(**locals()))