Exception when running "paster front-end-build"

```
paster front-end-build                                                                                                           master 
compile green.css
compile fuchsia.css
compile maroon.css
compile red.css
compile main.css
Generating hu
Generating sq
Generating de
Generating fr
Generating nl
Generating pt_BR
Generating el
Generating ro
Generating it
Generating sl
Generating ja
Generating no
Generating ca
Generating sk
Generating lv
Generating sr_Latn
Generating es
Generating sv
Generating cs_CZ
Generating lt
Generating bg
Generating ru
Generating sr
Generating fi
Generating pl
Completed generating JavaScript translations
Traceback (most recent call last):
  File "/home/adria/dev/pyenvs/ckan_plain/bin/paster", line 9, in <module>
    load_entry_point('PasteScript==1.7.5', 'console_scripts', 'paster')()
  File "/home/adria/dev/pyenvs/ckan_plain/local/lib/python2.7/site-packages/paste/script/command.py", line 104, in run
    invoke(command, command_name, options, args[1:])
  File "/home/adria/dev/pyenvs/ckan_plain/local/lib/python2.7/site-packages/paste/script/command.py", line 143, in invoke
    exit_code = runner.run(args)
  File "/home/adria/dev/pyenvs/ckan_plain/local/lib/python2.7/site-packages/paste/script/command.py", line 238, in run
    result = self.command()
  File "/home/adria/dev/pyenvs/ckan_plain/src/ckan/ckan/lib/cli.py", line 1951, in command
    cmd.command()
  File "/home/adria/dev/pyenvs/ckan_plain/src/ckan/ckan/lib/cli.py", line 1784, in command
    if self.options.clean:
AttributeError: Values instance has no attribute 'clean'
```