Use Qt4 by default instead of Qt5

Earlier version of VTK does not support Qt5 and PCL will not compile
correctly when Qt5 is used by default.
From VTK wiki (http://www.vtk.org/Wiki/VTK/Configure_and_Build):
```
Building VTK with Qt is a common use case for the creation of nice
user interfaces. Using Qt 4.8.* is recommended, but earlier versions
and Qt 5.* are in various stages of support.
...
Qt5.*
Warning: This has only been tested on Qt5.2.1, and only on a few systems.
```
Merge pull request #765 from xiangxw/qt4_default

Use Qt4 by default instead of Qt5