Drop support for Java 6 (#606)

diff --git a/build.gradle b/build.gradle
index 57018e6..431fad3 100644
--- a/build.gradle
+++ b/build.gradle
@@ -166,8 +166,8 @@
     }
 
     if (!androidProject) {
-        sourceCompatibility = JavaVersion.VERSION_1_6
-        targetCompatibility = JavaVersion.VERSION_1_6
+        sourceCompatibility = JavaVersion.VERSION_1_7
+        targetCompatibility = JavaVersion.VERSION_1_7
 
         [compileJava, compileTestJava].each() {
             it.options.compilerArgs += ["-Xlint:all", "-Xlint:-options", '-Xmaxwarns', '9999999']