Append SNAPSHOT classifier to version

Also increase the version from [1/5].6.0 to [1/5].6.1 which
results in 1.6.1-SNAPSHOT for the Platform and 5.6.1-SNAPSHOT
for Jupiter and Vintage modules.

gradle.properties

@@ -1,13 +1,13 @@
 group           = org.junit
-version         = 5.6.0
+version         = 5.6.1-SNAPSHOT
 
 jupiterGroup    = org.junit.jupiter
 
 platformGroup   = org.junit.platform
-platformVersion = 1.6.0
+platformVersion = 1.6.1-SNAPSHOT
 
 vintageGroup    = org.junit.vintage
-vintageVersion  = 5.6.0
+vintageVersion  = 5.6.1-SNAPSHOT
 
 defaultBuiltBy  = JUnit Team
 releaseBranch   = master