Change Java build target from 1.6 to 7

JDK 12 and higher only support targeting Java 7 and higher.

build.gradle

@@ -13,8 +13,8 @@ wrapper {
   distributionType = Wrapper.DistributionType.ALL
 }
 
-sourceCompatibility = 1.6
-targetCompatibility = 1.6
+sourceCompatibility = 7
+targetCompatibility = 7
 
 repositories {
   mavenLocal()