Configure black to use pipenv (braintreeps#138)

Since we're already checking for a Pipfile, we can tell the plugin to
run Black via pipenv. This prevents having to also install Black outside
the virtualenv.

vimrc

@@ -167,6 +167,7 @@ let g:ale_fix_on_save = 1
 let black = system('grep -q black Pipfile')
 if v:shell_error == 0
   let g:ale_fixers['python'] = ['black']
+  let g:ale_python_black_auto_pipenv = 1
 endif
 
 let html_use_css=1