Begin development on v6.0.0

Change-Id: Ie614694c61d5f02c19e818eccb42dc4627417e0b

Author: samtstern

buildSrc/src/main/kotlin/Config.kt

@@ -1,5 +1,5 @@
 object Config {
-    const val version = "5.1.0"
+    const val version = "6.0.0-SNAPSHOT"
     val submodules = listOf("auth", "auth-github", "common", "firestore", "database", "storage")
 
     private const val kotlinVersion = "1.2.61"