Fix current SNAPSHOT version. (#981)

Mea culpa, I set it wrongly during the last release.
diff --git a/build.gradle b/build.gradle
index 0760e9b..162c491 100644
--- a/build.gradle
+++ b/build.gradle
@@ -51,7 +51,7 @@
 
     group = "org.conscrypt"
     description = 'Conscrypt is an alternate Java Security Provider that uses BoringSSL'
-    version = "2.5.0-SNAPSHOT"
+    version = "2.6-SNAPSHOT"
 
     ext {
         os = org.gradle.internal.os.OperatingSystem.current();