Print files with unapproved licenses in travis

[jihoonson]

This PR shows the list of files with unapproved licenses after running Apache rat in Travis, so that we don't have to run again manually to see them.

The below is an example output. I tested with #6923.

...
[INFO] 118 resources included (use -debug for more details)
[WARNING] Files with unapproved licenses:
  /Users/jihoonson/Codes/druid/web-console/legacy/druid.css
  /Users/jihoonson/Codes/druid/web-console/legacy/druid.js
  /Users/jihoonson/Codes/druid/web-console/lib/react-table.styl
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for Druid 0.13.0-incubating-SNAPSHOT:
[INFO] 
[INFO] Druid .............................................. SUCCESS [  6.880 s]
[INFO] druid-core ......................................... SUCCESS [ 24.825 s]
[INFO] druid-hll .......................................... SUCCESS [  5.012 s]
[INFO] extendedset ........................................ SUCCESS [  4.893 s]
[INFO] druid-processing ................................... SUCCESS [ 25.595 s]
[INFO] druid-aws-common ................................... SUCCESS [  4.531 s]
[INFO] druid-console ...................................... FAILURE [ 32.699 s]
...

[gianm]

LGTM after CI