stack/slapos: Pin slapos.extension.strip

For builds to be reproducible, we pin versions for used eggs and
buildout extensions, but 30d43f03 (use slapos.extension.strip to strip
binaries at the end of buildout process.) forgot to do this.

/cc @kazuhiko

stack/slapos.cfg

@@ -135,6 +135,7 @@ simplejson = 3.6.5
 six = 1.9.0
 slapos.cookbook = 0.101
 slapos.core = 1.3.10
+slapos.extension.strip = 0.1
 slapos.libnetworkcache = 0.14.2
 slapos.recipe.build = 0.21
 slapos.recipe.cmmi = 0.2