Fix PermGen OOME

*Why I did it?*
When running multiple tests with RoboSpecification, perm size increases drastically.
For ~10 specifications it can cause OutOfMemory errors. 

*How I did it?*
Large perm usage was due to creating separated classloader for every
specification. Create RobolectricClassLoader as static field,
initialize it only once reuse it among RoboSpecifications.