Update groupId spark --> com.sparkjava

In preparation for using Sonatype OSS for sync to the Maven central repository, the docs say

IMPORTANT the groupId of your projects, generally it must match your domain name, so com.googlecode.myprj is valid but myprj is not, if you are unsure about this, please read Choosing your Coordinates

https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide
https://docs.sonatype.org/display/Repository/Choosing+your+Coordinates

There are also several other projects already in Maven central called Spark

http://search.maven.org/#search|ga|1|spark


This is a small file change but has significant downstream effects -- you'll need to update the docs at sparkjava.com, users already on 0.9.9.4-SNAPSHOT will have to change their pom.xml, etc.