Fix generating the javadoc. (#144)

build.gradle

@@ -34,7 +34,7 @@ buildscript {
         maven { url "https://plugins.gradle.org/m2/" }
     }
     dependencies {
-        classpath "com.google.protobuf:protobuf-gradle-plugin:0.8.5"
+        classpath "com.google.protobuf:protobuf-gradle-plugin:0.8.6"
     }
 }
 
@@ -50,6 +50,8 @@ dependencies {
     compile "com.google.protobuf:protobuf-java:${protobufVersion}",
             "io.grpc:grpc-protobuf:${grpcVersion}",
             "io.grpc:grpc-stub:${grpcVersion}"
+
+    compileOnly "javax.annotation:javax.annotation-api:1.2"
 }
 
 protobuf {
@@ -96,6 +98,8 @@ signing {
     sign configurations.archives
 }
 
+javadoc.source = "$projectDir/gen_gradle/src"
+
 javadoc.options {
     encoding = 'UTF-8'
     links 'https://docs.oracle.com/javase/8/docs/api/'