TAJO-890: Redirect stdout of maven test to /dev/null in Travis CI script.

Closes #47
TAJO-866: COUNT DISTINCT with other aggregation function throws ClassCastException. (Hyoungjun Kim via hyunsik)

Closes #36
TAJO-874: Sometimes InvalidOperationException occurs when aggregates TableStat. (Hyoungjun Kim via hyunsik)
TAJO-874: Sometimes InvalidOperationException occurs when aggregates TableStat. (Hyoungjun Kim via hyunsik)

Closes #38