Start 1.15.0 development cycle (#4650)

diff --git a/build.gradle b/build.gradle
index 4c25986..5a01e6e 100644
--- a/build.gradle
+++ b/build.gradle
@@ -51,7 +51,7 @@
     idea.module.inheritOutputDirs = true
 
     group = "io.grpc"
-    version = "1.14.0-SNAPSHOT" // CURRENT_GRPC_VERSION
+    version = "1.15.0-SNAPSHOT" // CURRENT_GRPC_VERSION
 
     sourceCompatibility = 1.6
     targetCompatibility = 1.6