Different dashboard for master and stable builds

[jcfr (Jean Christophe Fillion Robin (Kitware))]

Updates:

• Dashboard will be SlicerPreview

• Slicer has been updated accordingly - see r26928

• New PR mhalle/slicer4-download#13 was just submitted to update http://download.slicer.org/ and use the term Preview Release instead of Nightly Build