Bump version to 3.12.6-SNAPSHOT

Author: jyemin

build.gradle

@@ -54,7 +54,7 @@ def javaCodeCheckedProjects = subprojects.findAll { !['util', 'mongo-java-driver
 configure(coreProjects) {
     evaluationDependsOn(':util')
     group = 'org.mongodb'
-    version = '3.12.5'
+    version = '3.12.6-SNAPSHOT'
 
     repositories {
         mavenLocal()