requirements: Add missing chardet dependency used by npm loader

It was previously installed through a transient requirement that
removed its use so it must be explicitly listed in the requirements.

requirements.txt

@@ -2,6 +2,7 @@
 # should match https://pypi.python.org/pypi names. For the full spec or
 # dependency lines, see https://pip.readthedocs.org/en/1.1/requirements.html
 
+chardet
 psutil
 requests
 iso8601