Fix defaultTemplatePath -- WEB-INF was not present

After reading a lot of source code trying to make my Scalatra app run on Tomcat,
I noticed that the suggested defaultTemplatePath was not right. As you can see in
[1], it should start with /WEB-INF.

I'm not sure whether the same is true for DefaultLayoutStrategy, but I believe so.

[1] https://github.com/scalatra/scalatra/blob/develop/scalate/src/main/scala/org/scalatra/scalate/ScalateSupport.scala#L169-L170