Fix gradle build file to declare proper version of Java

complete/build.gradle

@@ -21,6 +21,9 @@ repositories {
     mavenCentral()
 }
 
+sourceCompatibility = 1.7
+targetCompatibility = 1.7
+
 dependencies {
     compile("org.springframework.boot:spring-boot-starter-web")
     // tag::actuator[]

initial/build.gradle

@@ -21,6 +21,9 @@ repositories {
     mavenCentral()
 }
 
+sourceCompatibility = 1.7
+targetCompatibility = 1.7
+
 dependencies {
     // tag::jetty[]
     compile("org.springframework.boot:spring-boot-starter-web") {