Update to 1.6.

Disable the new mapnik option, in order to separate the upgrade work
from the mapnik work, not because it's undesirable.

>From NEWS:

Viking 1.6 (2015-03-29)
New features since 1.5.1
* Mapnik Rendering Layer (not available in Windows build ATM) - with
  preprocessor CartoCSS option.
** Build system dependency on C++. Source dependency on libmapnik-dev.
* Export files via GPSBabel
* Extended GPSBabel filtering support - Enables limited/simple OSM Overpass
   API support.
* Support reading of MBTiles files.
* Support reading of OSM Metatiles.
* Configurable Toolbar
* Configurable Menu keys
* Various time related features:
** Edit trackpoint and waypoint times.
** Sort layers by time.
** Preference option 'World' to display time values based on the position
   (i.e. in the local timezone) of the track or waypoint.
** Show time axis values on time graphs
** Quick search of tracks by date
** Open external programs Rednotebook or Stellarium at the specific
   time (and place) of a point.
* Improved route finder UI usage
* GeoJSON support via external programs 'togpx' and 'togeojson'
* New default Map Cache layout to be more compatible with standard
  OSM/TMS structure.
** Also python tool available to migrate cache layouts
* Support GPX Metadata values common to GPX1.0 and GPX1.1
* Extended map type configuration options (zoom levels + area extents limits)
* Build compatibility with GPSD 3.12
* Allow starting Viking with command line parameters to set position, zoom
  and map type.
** --latitude <degrees>
** --longitude <degrees>
** --zoom <ZoomLevelOSM>
** --map <MapID>
** eg: $> viking --latitude 51.4 --longitude -1.3 --zoom 12 --map 13
* Improved Windows install information and options
* Export Track, Route and Waypoint Lists as text.
* Many internal improvements, fixes, more translations and various other small
  improvements.


(gdt)