Specify Java source and target compatibility version

Add the directive to specify the version of Java to use
to compile the Java files. Previously the version was not specified
so the local JDK was used.

This caused a problem with version 1.1.0 of the plugin that got compiled
with Java 11 and was incompatible for users on Java 8.

Fixes #38

Author: cortinico

README.md

@@ -10,7 +10,6 @@ This plugin wraps [swagger-codegen](https://github.com/swagger-api/swagger-codeg
 
 **Swagger Gradle Codegen** is distributed through [Gradle Plugin Portal](https://plugins.gradle.org/). To use it you need to add the following dependency to your gradle files.
 Please note that those code needs to be added the gradle file of the module where you want to generate the code (**not the top level** build.gradle\[.kts\] file).
-Please also note that the plugin uses gradle 5.+ features, and you will need to setup your gradle wrapper to use 5.0 or more.
 
 If you're using the Groovy Gradle files:
 ```groovy
@@ -53,6 +52,13 @@ Please note that the `generateSwagger { }` block is **needed** in order to let t
 
 Once you setup the plugin correctly, you can call the `:generateSwagger` gradle task, that will run the code generation with the configuration you provided.
 
+## Requirements
+
+In order to run this gradle plugin you need to fulfill the following requirements:
+
+* Gradle 5.x - This plugin uses Gradle 5 features, and you will need to setup your Gradle wrapper to use 5.0 or more.
+* Java 8+
+
 ## Supported platforms
 
 The Swagger Gradle Codegen is designed to support multiple platforms. For every platform, we provide **templates** that are tested and generates opinionated code.

plugin/build.gradle.kts

@@ -11,6 +11,11 @@ plugins {
     id("com.gradle.plugin-publish") version "0.10.0"
 }
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+}
+
 dependencies {
     implementation(kotlin("stdlib-jdk8"))