make set_version.sh work on mac

set_version.sh

@@ -1,7 +1,10 @@
 #!/bin/bash
+#
+# Called by jenkins to update version.rs with the current build number
+#
 
 if [[ -z "${BUILD_NUMBER}" ]] ; then
   echo "No BUILD_NUMBER environment variable was set"
   exit 1
 fi
-sed -i "s/.DEV_BUILD/.${BUILD_NUMBER}/" src/version.rs
+sed -i "s/.DEV_BUILD/\.${BUILD_NUMBER}/" src/version.rs