travis: switch to pip for fetching the genmsg dependencies.

Ubuntu Precise doesn't have a pre-packaged version of catkin.

Signed-off-by: Michael Hope <mlhx@google.com>

.travis.yml

@@ -29,7 +29,8 @@ install:
   - sudo apt-get install build-essential git libc6-i386
   - tar -xf gcc-arm-none-eabi-4_8-2014q3-20140805-linux.tar.bz2
   - export PATH=$PATH:$PWD/gcc-arm-none-eabi-4_8-2014q3/bin
-  - sudo apt-get install -qq python-catkin-pkg python-empy
+  - sudo apt-get install -qq python-pip
+  - sudo pip install -q empy catkin_pkg
   - git clone https://github.com/ros/genmsg
   - cd genmsg && sudo python setup.py install && cd ..