Fix pom.xml in jackrabbit core (small fix, big return)

Change the following dependency in pom.xml to match what is really available at the maven repos:
FROM:
<dependency>
<groupId>jsr170</groupId>
<artifactId>jcr</artifactId>
<version>1.0</version>
</dependency>

TO:
<dependency>
<groupId>javax.jcr</groupId>
<artifactId>jcr</artifactId>
<version>1.0</version>
</dependency>