Build failure when WITH_PCL=off and WITH_STUBS=on

When I build with clang-503.0.40 on x86_64-apple-darwin13.3.0 WITH_PCL=off but WITH_STUBS=on, I get the following ([full output](https://gist.github.com/gadomski/54424539d6505e06555b)):

> ../src/../include/pdal/PCLConversions.hpp:111:22: error: no member named 'has_color' in namespace 'pcl::traits'

Two problems really:

- This problem snuck through Travis because we set WITH_STUBS=off [in our CI config](https://github.com/PDAL/PDAL/blob/master/scripts/ci/script.sh#L44) — should this be on when we're testing with `PDAL_OPTIONAL_COMPONENTS=none`? My background on the motivation behind the stubs is nil so don't know the right answer here.

- Then there's the main issue, which is the missing declaration of `has_color`. I found a [definition](https://github.com/PointCloudLibrary/pcl/blob/master/common/include/pcl/point_types.h#L746) of `has_color` in the PCL master branch, but I'm not familiar enough with PCL to know whether we should define that in our stubs, or what.