Due to Oracle license restriction, there are no public repositories that provide ojdbc jar.
You need to download it and install in your local repository.
Get jar from Oracle and install it in your local maven repository using
mvn install:install-file -Dfile={path/to/your/ojdbc.jar} -DgroupId=com.oracle
-DartifactId=ojdbc6 -Dversion=11.2.0 -Dpackaging=jar
If you are using ojdbc7, here is the link