support also ojdbc6.jar for Java 1.7

This pull request supports ojdbc6.jar for Java 1.7.

Since #335 ojdbc7.jar is required if you runs Java 1.7. Though Oracle supports ojdbc6.jar for Java 1.7 
if your ojdbc6.jar version is 11.2.0.3 or 11.2.0.4.


in case of Java 1.7 check for both ojdbc7.jar and ojdbc6.jar file in application ./lib directory, in load path and in environment PATH variable

Thanks @rsim