docs(node): change to at least node 8.9.4 (#138)

For the rerun only affected tests on saving a test or src file the node
runner needs at least node version `8.9.4` else we will not be able to
find the corresponding module in the `require` cache

README.md

@@ -10,7 +10,7 @@
 
 ### Default runner
 
-* [node](./docs/node.md#node): Node.js > 8
+* [node](./docs/node.md#node): Node.js >= 8.9.4
 
 ### Context dependent