build.gradle: Make protobuf-gradle-plugin use protoc from the reposit…

…ory.

This updates the protoc version to 3.6.1, fixing many warnings in the generated sources.

.travis.yml

@@ -5,8 +5,7 @@ language: java
 jdk:
   - openjdk8
   - openjdk11
-install:
-  - sudo apt-get install -y protobuf-compiler
+install: true
 script:
     # excluding wallet-template because of the unreliable JavaFX dependency
   - gradle clean build -x :wallettemplate:compileJava

core/build.gradle

@@ -29,6 +29,9 @@ compileJava.options.encoding = 'UTF-8'
 compileTestJava.options.encoding = 'UTF-8'
 
 protobuf {
+    protoc {
+        artifact = 'com.google.protobuf:protoc:3.6.1'
+    }
     generatedFilesBaseDir = new File(projectDir, '/src') // workaround for '$projectDir/src'
 }