Update pom.xml

"mvn initialize" fails with maven 3.2.1 due resolve dependency to "org.grails:grails-dependencies:jar:2.3.8". Since this depency is only distributed as pom and not as jar. After added type with value pom to dependency "grails-dependencies" so maven 3.2.1 is able to build correctly.

fixes [https://jira.grails.org/browse/MAVEN-218]