Use latest confirmed working version of urdf_parser_py

Using urdf_parser_py's commit 31474b9baaf7c3845b40e5a9aa87d5900a2282c3,
that precedes the regression introduced in
ros/urdf_parser_py#26 (ros/urdf_parser_py#26)
and discussed in
robotology/simmechanics-to-urdf#36 (robotology/simmechanics-to-urdf#36).
The commit used is based from the info contained in this icub-models commit:
robotology/icub-models@b69b989 (robotology/icub-models@b69b989).

.travis.yml

@@ -48,6 +48,7 @@ before_script:
   # install urdf_parser_py and save the last commit SHA1 hash
   - git clone $URDF_PARSER_PY_REPOSITORY_URL
   - cd urdf_parser_py
+  - git checkout 31474b9baaf7c3845b40e5a9aa87d5900a2282c3
   - export URDF_PARSER_PY_COMMIT=`git rev-parse HEAD`
   - sudo python setup.py install
   - cd ../