Improve procedure for creating release tag and branch

The flow itself was improved, but also the addition of Bash variables
and sed automation should hopefully make it less error-prone.
diff --git a/build.gradle b/build.gradle
index e325464..8f36af8 100644
--- a/build.gradle
+++ b/build.gradle
@@ -19,7 +19,7 @@
     apply plugin: "com.google.osdetector"
 
     group = "io.grpc"
-    version = "0.14.0-SNAPSHOT"
+    version = "0.14.0-SNAPSHOT" // CURRENT_GRPC_VERSION
 
     sourceCompatibility = 1.6
     targetCompatibility = 1.6