Relax spyder dependency
Installing spyder>=3 can be a problem in older systems (e.g. debian 8) because it forces many dependencies (e.g. ipython>4) which may not be wanted. On the other hand, spyder is only needed for taurus.qt.editor.
Allow importing taurus.qt.editor even if spyder is not installed. In that case, return a dummy widget and log a warning. Also make the extras_requires target for spyder more specific.
This will fix the errors seen in the travis builds since #554 was merged.