Update pom.xml
Update pom.xml

extra memory for surefire, as travis is running into a `java.lang.OutOfMemoryError: PermGen space` 

see: https://travis-ci.org/GeoDienstenCentrum/sass-maven-plugin/jobs/53167402#L6700
GeoDienstenCentrum/sass-maven-plugin,GeoDienstenCentrum/sass-maven-plugin