Proper release of NEST 3.6

Moving from release candidate 2 to the actual release, no further issues showed up yet.

packages/nest/package.py

@@ -28,7 +28,7 @@ class Nest(CMakePackage):
     maintainers = ['terhorst']
 
     version('master', branch='master')
-    version('3.6-rc2',sha256='8508a9590ad49e6657809d1acfd02aa612b70fa8e5e7791a2ea261d891f181f7')
+    version('3.6',    sha256='68d6b11791e1284dc94fef35d84c08dd7a11322c0f1e1fc9b39c5e6882284922')
     version('3.5',    sha256='3cdf5720854a4d8a7d359f9de9d2fb3619a0be2e36932028d6940360741547bd')
     version('3.4',    sha256='c56699111f899045ba48e55e87d14eca8763b48ebbb3648beee701a36aa3af20')
     version('3.3',    sha256='179462b966cc61f5785d2fee770bc36f86745598ace9cd97dd620622b62043ed')