python: Remove --upgrade from pip install

It breaks older pip versions (Ubuntu 12.04).

python/manifests/init.pp

@@ -183,7 +183,7 @@ define python::pip::install($source=undef) {
     exec { "pip-install-${name}":
       refreshonly => true,
       path        => '/bin:/usr/bin:/usr/local/bin:/sbin:/usr/sbin:/usr/local/sbin',
-      command     => "pip install --no-index --upgrade /usr/local/src/${name}",
+      command     => "pip install --no-index /usr/local/src/${name}",
       subscribe   => File["/usr/local/src/${name}"],
     }
   } else {