OS X Yosemite (10.10) Rosdep entries for vtk and gazebo2

#### Homebrew VTK Update
Recent updates to the homebrew/science/vtk brew have created issues for ROS Indigo install on OSX. 
This can be fixed by removing the __--python__ and __--qt__ options and replacing them with __--with-qt__ on the vtk homebrew dependency.

#### ROS & Gazebo
As noted on the [Which combination of ROS/Gazebo versions to use](http://gazebosim.org/tutorials?tut=ros_wrapper_versions&cat=connect_ros) page, ROS Indigo is only compatible with the Gazebo 2.x branch.  
The problem with this change is that hydro still needs gazebo1.x. If I mere this then anyone who resolves the key "gazebo" on OS X will get gazebo2, even people trying to install hydro.

Unfortunately there are not a lot of good options here, but we can:

- Have packages update their dependencies to be `gazebo2` instead of `gazebo` in Indigo
- Have rosdep install one or the other based on the ROS distro

Neither are good solutions since the first requires changing several packages and maintaining at least two rosdep rules for gazebo. And the second on e is also not great mostly because rosdep does not support that type of rule, where one thing is installed for one ROS distro and another for a different one.

I think the "right" thing to do is to have another rosdep rule "gazebo2" which points to gazebo2 in Homebrew. Then we should update the packages in ROS which directly depend on gazebo to directly depend on gazebo2 instead. This shouldn't be too bad since most people get to gazebo through the `gazebo_ros_pkgs` package.

Regardless of the final solution I don't merge this change as-is makes sense.