Fix typo MemmapingPool in rllab

Error message:

    using seed 3
    Traceback (most recent call last):
      File "/root/rllab/scripts/run_experiment_lite.py", line 137, in <module>
        run_experiment(sys.argv)
      File "/root/rllab/scripts/run_experiment_lite.py", line 74, in run_experiment
        from rllab.sampler import parallel_sampler
      File "/root/rllab/rllab/sampler/parallel_sampler.py", line 2, in <module>
        from rllab.sampler.stateful_pool import singleton_pool, SharedGlobal
      File "/root/rllab/rllab/sampler/stateful_pool.py", line 3, in <module>
        from joblib.pool import MemmapingPool
    ImportError: cannot import name 'MemmapingPool'

Reference: rll/rllab#240

Dockerfile

@@ -89,3 +89,4 @@ RUN MUJOCO_ZIP="mjpro$(echo ${MUJOCO_VERSION} | sed -e "s/\.//g")_linux.zip" \
 # ========== Package Patches ==========
 
 RUN source activate sql && pip install "numpy>=1.16"
+RUN sed -i 's/MemmapingPool/MemmappingPool/g' /root/rllab/rllab/sampler/stateful_pool.py