Add a configuration option to control the type of colcon-bundle installation (from pip or source)

[AAlon]

We're currently installing colcon bundle from source due to an issue with the version that exists in pip.

Once colcon bundle is released to pip (along with the following commit: colcon/colcon-bundle@d5ea60e) we should remove the installation from source.

Related to PR #35

[AAlon]

After discussing with @raghaprasad, we agreed the path forward should be to support an additional build job which would use colcon-bundle from source. So rather than removing the installation from source, we should guard it inside an if condition and provide that flag via .travis.yml.
Changing the title accordingly.