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

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