stack/slapos.cfg: wheel = 0.35.1:whl

This is a workaround to avoid the UnicodeDecodeError that occurs
when installing wheel, in particular during compilation of ERP5.

stack/slapos.cfg

@@ -203,7 +203,7 @@ stevedore = 1.21.0:whl
 subprocess32 = 3.5.4
 unicodecsv = 0.14.1
 wcwidth = 0.2.5
-wheel = 0.35.1
+wheel = 0.35.1:whl
 xml-marshaller = 1.0.2
 zc.lockfile = 1.0.2
 zdaemon = 4.2.0