add init target to update gradlew based on user gradle version

Author: jvmvik

Makefile

@@ -1,11 +1,16 @@
 .PHONY: build
 
-all: clean build test readme doc
+all: clean init build test readme doc
 
 # clean
 clean:
 	./gradlew clean
 
+# Initialize gradle wrapper
+init:
+	gradle wrapper
+	chmod +x ./gradlew
+
 # Run gradle test with information
 test:
 	./gradlew test --info