Make hook more portable (expr didn't support substr on OS X)

theano/misc/hg_version_hook.sh

@@ -5,6 +5,6 @@ if [ -z "$HG_NODE" ] ; then
     echo No HG_NODE, skipping update of theano.__version__
     exit 0 # this seems to be normal sometimes
 else
-    sed -e "s/^hg_revision.*/hg_revision = '`expr substr $HG_NODE 1 12`'/" theano/version.py >theano/version.py.out && mv theano/version.py.out theano/version.py
+    sed -e "s/^hg_revision.*/hg_revision = '`python -c \"print \\"$HG_NODE\\"[0:12]\"`'/" theano/version.py >theano/version.py.out && mv theano/version.py.out theano/version.py
 fi