Update the Travis CI config to output log files on test failures

This change updates the Travis CI configuration file to output the
contents of the PyKMIP server and SLUGS log files if:

* a test failure occurred, and
* the log files exist in the test environment

This makes debugging server and service failures much easier in
Travis CI, specifically during integration and functional test
suite runs.

.travis.yml

@@ -232,3 +232,7 @@ script:
   - ./.travis/run.sh
 after_success:
   - codecov
+after_failure:
+  - if [ -f "/tmp/pykmip/server.log" ]; then cat /tmp/pykmip/server.log; fi
+  - if [ -f "/tmp/slugs/access.log"  ]; then cat /tmp/slugs/access.log;  fi
+  - if [ -f "/tmp/slugs/error.log"   ]; then cat /tmp/slugs/error.log;   fi