[SPARK-9507] [BUILD] Remove dependency reduced POM hack now that shade plugin is updated

Update to shade plugin 2.4.1, which removes the need for the dependency-reduced-POM workaround and the 'release' profile. Fix management of shade plugin version so children inherit it; bump assembly plugin version while here

See https://issues.apache.org/jira/browse/SPARK-8819

I verified that `mvn clean package -DskipTests` works with Maven 3.3.3.

pwendell are you up for trying this for the 1.5.0 release?

Author: Sean Owen <sowen@cloudera.com>

Closes #7826 from srowen/SPARK-9507 and squashes the following commits:

e0b0fd2 [Sean Owen] Update to shade plugin 2.4.1, which removes the need for the dependency-reduced-POM workaround and the 'release' profile. Fix management of shade plugin version so children inherit it; bump assembly plugin version while here