ARROW-2097: [CI, Python] Reduce Travis-CI verbosity
ARROW-2097: [CI, Python] Reduce Travis-CI verbosity

Python tests with Valgrind enabled produce very long output in verbose mode.

Author: Antoine Pitrou <antoine@python.org>

Closes #1883 from pitrou/ARROW-2097-reduce-pytest-verbosity and squashes the following commits:

588917e <Antoine Pitrou> ARROW-2097:  Reduce Travis-CI verbosity

apache/arrow,pcmoritz/arrow,xhochy/arrow,renesugar/arrow,pcmoritz/arrow,pcmoritz/arrow,apache/arrow,itaiin/arrow,xhochy/arrow,pcmoritz/arrow,wesm/arrow,itaiin/arrow,apache/arrow,apache/arrow,apache/arrow,pcmoritz/arrow,wagavulin/arrow,wagavulin/arrow,renesugar/arrow,cpcloud/arrow,itaiin/arrow,wagavulin/arrow,apache/arrow,cpcloud/arrow,apache/arrow,xhochy/arrow,kou/arrow,xhochy/arrow,renesugar/arrow,pcmoritz/arrow,laurentgo/arrow,renesugar/arrow,icexelloss/arrow,kou/arrow,wagavulin/arrow,majetideepak/arrow,icexelloss/arrow,kou/arrow,itaiin/arrow,xhochy/arrow,xhochy/arrow,apache/arrow,wagavulin/arrow,majetideepak/arrow,icexelloss/arrow,itaiin/arrow,wesm/arrow,laurentgo/arrow,wesm/arrow,xhochy/arrow,xhochy/arrow,kou/arrow,kou/arrow,tebeka/arrow,icexelloss/arrow,icexelloss/arrow,majetideepak/arrow,kou/arrow,wagavulin/arrow,xhochy/arrow,itaiin/arrow,itaiin/arrow,cpcloud/arrow,pcmoritz/arrow,apache/arrow,laurentgo/arrow,kou/arrow,renesugar/arrow,majetideepak/arrow,renesugar/arrow,itaiin/arrow,majetideepak/arrow,itaiin/arrow,cpcloud/arrow,wesm/arrow,wesm/arrow,itaiin/arrow,itaiin/arrow,majetideepak/arrow,tebeka/arrow,wesm/arrow,apache/arrow,apache/arrow,majetideepak/arrow,tebeka/arrow,laurentgo/arrow,majetideepak/arrow,pcmoritz/arrow,majetideepak/arrow,tebeka/arrow,laurentgo/arrow,pcmoritz/arrow,icexelloss/arrow,laurentgo/arrow,xhochy/arrow,renesugar/arrow,cpcloud/arrow,itaiin/arrow,laurentgo/arrow,wesm/arrow,wagavulin/arrow,tebeka/arrow,laurentgo/arrow,majetideepak/arrow,wagavulin/arrow,laurentgo/arrow,cpcloud/arrow,icexelloss/arrow,xhochy/arrow,wagavulin/arrow,tebeka/arrow,majetideepak/arrow,apache/arrow,cpcloud/arrow,kou/arrow,majetideepak/arrow,laurentgo/arrow,renesugar/arrow,wesm/arrow,pcmoritz/arrow,laurentgo/arrow,pcmoritz/arrow,majetideepak/arrow,tebeka/arrow,cpcloud/arrow,renesugar/arrow,xhochy/arrow,icexelloss/arrow,apache/arrow,tebeka/arrow,wesm/arrow,pcmoritz/arrow,wesm/arrow,renesugar/arrow,tebeka/arrow,apache/arrow,cpcloud/arrow,majetideepak/arrow,cpcloud/arrow,renesugar/arrow,kou/arrow,cpcloud/arrow,itaiin/arrow,wesm/arrow,laurentgo/arrow,cpcloud/arrow,cpcloud/arrow,icexelloss/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,laurentgo/arrow,renesugar/arrow,itaiin/arrow,renesugar/arrow,icexelloss/arrow,wesm/arrow,pcmoritz/arrow,cpcloud/arrow,laurentgo/arrow,wagavulin/arrow,wagavulin/arrow,kou/arrow,pcmoritz/arrow,wagavulin/arrow,xhochy/arrow,icexelloss/arrow,xhochy/arrow,kou/arrow,wesm/arrow,wesm/arrow,icexelloss/arrow,kou/arrow,renesugar/arrow,xhochy/arrow,tebeka/arrow,majetideepak/arrow,pcmoritz/arrow,itaiin/arrow,kou/arrow,renesugar/arrow,kou/arrow