update proportion of memory

The default value of "spark.storage.memoryFraction" has been changed from 0.66 to 0.6 . So it should be 60% of the memory to cache while 40% used for task execution.

Author: Chen Chao <crazyjvm@gmail.com>

Closes #66 from CrazyJvm/master and squashes the following commits:

0f84d86 [Chen Chao] update proportion of memory

docs/tuning.md

@@ -163,8 +163,8 @@ their work directories), *not* on your driver program.
 **Cache Size Tuning**
 
 One important configuration parameter for GC is the amount of memory that should be used for caching RDDs.
-By default, Spark uses 66% of the configured executor memory (`spark.executor.memory` or `SPARK_MEM`) to
-cache RDDs. This means that 33% of memory is available for any objects created during task execution.
+By default, Spark uses 60% of the configured executor memory (`spark.executor.memory` or `SPARK_MEM`) to
+cache RDDs. This means that 40% of memory is available for any objects created during task execution.
 
 In case your tasks slow down and you find that your JVM is garbage-collecting frequently or running out of
 memory, lowering this value will help reduce the memory consumption. To change this to say 50%, you can call