Drop support for the low-level REST client on JDK 7

This commit bumps the minimum compiler version on the low-level REST
client to JDK 8.

Author: jasontedor

client/rest/build.gradle

@@ -22,8 +22,8 @@ apply plugin: 'elasticsearch.build'
 apply plugin: 'nebula.maven-base-publish'
 apply plugin: 'nebula.maven-scm'
 
-targetCompatibility = JavaVersion.VERSION_1_7
-sourceCompatibility = JavaVersion.VERSION_1_7
+targetCompatibility = JavaVersion.VERSION_1_8
+sourceCompatibility = JavaVersion.VERSION_1_8
 
 group = 'org.elasticsearch.client'
 archivesBaseName = 'elasticsearch-rest-client'