Updating SimpleITK version to v0.8.1

Change-Id: If82cfa8121da03fcddecade453a147c2e6e26ae5
Merge branch 'release'

Change-Id: I70b63b248ab228d2997a84f2353d8cc9feb4726a
Merge branch 'master' into next

Change-Id: I2300f12bb598cdfe1f134640f5cc569872f455f4