Set source and target compatibility to 1.7

in build.gradle to make it work with Android

modules/swagger-codegen/src/main/resources/Java/libraries/okhttp-gson/build.gradle.mustache

@@ -1,6 +1,9 @@
 apply plugin: 'java'
 apply plugin: 'maven'
 
+sourceCompatibility = 1.7
+targetCompatibility = 1.7
+
 repositories {
     mavenCentral()
 }