Gradle: Upgrade dependency versions. (#193)

build.gradle

@@ -25,16 +25,17 @@ jar.manifest {
             'Target-Compatibility': targetCompatibility)
 }
 
-def protobufVersion = '3.5.1'
-def protocVersion = '3.5.1'
-def grpcVersion = "1.14.0" // CURRENT_GRPC_VERSION
+def protobufVersion = '3.7.0'
+def protocVersion = '3.7.0'
+def grpcVersion = "1.19.0" // CURRENT_GRPC_VERSION
+def javaxAnnotationVersion = '1.3.2'
 
 buildscript {
     repositories {
         maven { url "https://plugins.gradle.org/m2/" }
     }
     dependencies {
-        classpath "com.google.protobuf:protobuf-gradle-plugin:0.8.6"
+        classpath "com.google.protobuf:protobuf-gradle-plugin:0.8.8"
     }
 }
 
@@ -51,7 +52,7 @@ dependencies {
             "io.grpc:grpc-protobuf:${grpcVersion}",
             "io.grpc:grpc-stub:${grpcVersion}"
 
-    compileOnly "javax.annotation:javax.annotation-api:1.2"
+    compileOnly "javax.annotation:javax.annotation-api:${javaxAnnotationVersion}"
 }
 
 protobuf {