update webjars-locator due to NPE

All instructions in the guide and in the Webjars documentation leads to NPE:

e.g.
```
[NullPointerException: null]
5        var require = {
6            callback: function() {
7            // default requirejs configs
8                @for(webJarJson <- org.webjars.RequireJS.getSetupJson(routes.WebJarAssets.at("").url).values()) {
```

It's probably related to

https://github.com/webjars/webjars-locator/issues/60