catkin: 0.6.8-0 in 'indigo/distribution.yaml' [bloom]

Increasing version of package(s) in repository `catkin` to `0.6.8-0`:

- distro file: `indigo/distribution.yaml`
- bloom version: `0.5.10`
- previous version for package: `0.6.7-0`

## catkin

```
* make nosetests --xunit-file argument an absolute path to work around nose bug 779 (#659 <https://github.com/ros/catkin/issues/659>)
* fix handling of CMake packages which do not install any files (#665 <https://github.com/ros/catkin/issues/665>)
* fix gtest on Arch Linux and others (#663 <https://github.com/ros/catkin/issues/663>)
* improve generation of .catkin marker file (#671 <https://github.com/ros/catkin/issues/671>, #676 <https://github.com/ros/catkin/issues/676>)
* escape messages to avoid CMake warning (#667 <https://github.com/ros/catkin/issues/667>)
* fix CMake warning for doxygen target with CMake 3 (#660 <https://github.com/ros/catkin/issues/660>)
* avoid using ARGN for efficiency (#669 <https://github.com/ros/catkin/issues/669>)
```