Account for PYTHON_LIBRARY and PYTHON_INCLUDE_DIR changing over time

[spmaniato]

In #40, PYTHON_LIBRARY and PYTHON_INCLUDE_DIR became part of catkin config. However, if Homebrew Python is updated (and then brew cleanup is invoked), those directories are replaced with the ones corresponding to the newer version. In this case, catkin build fails because the compiler cannot find various Python-related files, e.g. fatal error: 'Python.h' file not found

For example, before fixing the issue, I had Python 2.7.12:

$ brew info python
python: stable 2.7.12 (bottled), HEAD

but catkin / CMake was looking for 2.7.11

-DPYTHON_LIBRARY=/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/libpython2.7.dylib
-DPYTHON_INCLUDE_DIR=/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/include/python2.7

This won't show up for people using the install script even when they just want to update their installation, as the catkin config command will be re-run. However, I would argue that we should remove those two flags from the configuration. And only keep (i) -install --install-space ${ROS_INSTALL_DIR} and (ii) -DCMAKE_BUILD_TYPE=Release, as those won't ever change. If that makes sense, I can open a pull request.