Added a fix that worked for me with a complex maven build

I applied this change internally to 1.9.9, but since you are working on 1.10.0 I just copied it over.

We have a large, multi-module maven build. The directory structure looks like this:

/pom.xml
/main-code/pom.xml
/main-code/webapps/pom.xml
/main-code/webapps/our-new-grails-app/pom.xml

When running `mvn install` from the root, the asset pipeline was writing `target/assets` to the root folder instead of to `/main-code/webapps/our-new-grails-app`.