[SPARK-15796][CORE] Reduce spark.memory.fraction default to avoid overrunning old gen in JVM default config

## What changes were proposed in this pull request?

Reduce `spark.memory.fraction` default to 0.6 in order to make it fit within default JVM old generation size (2/3 heap). See JIRA discussion. This means a full cache doesn't spill into the new gen. CC andrewor14

## How was this patch tested?

Jenkins tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #13618 from srowen/SPARK-15796.