Simplify make_package_lines().

Leave generate_messages() as comment if no messages are in the package.
This makes it easier to add them later if needed.

PS:
In general one should distinguish between getting the data and building the
result string. A 'template' for the entire CMakeList.txt would be cleaner
than the current approach.
Fix test_catkinize_cmakelists().

Don't check the entire string but only the relevant parts. Also split the test
in two (the two possible cases) to make it easier to locate the problem.
Merge branch 'fix_unittests' into travis