all: Swap to Java 7 source and bytecode

Core and OkHttp are left with Java 6 for the moment. Once we resolve
their issues they could be bumped as well.

Updates #3961
diff --git a/build.gradle b/build.gradle
index 21d0016..e4d6140 100644
--- a/build.gradle
+++ b/build.gradle
@@ -55,8 +55,8 @@
     group = "io.grpc"
     version = "1.15.0-SNAPSHOT" // CURRENT_GRPC_VERSION
 
-    sourceCompatibility = 1.6
-    targetCompatibility = 1.6
+    sourceCompatibility = 1.7
+    targetCompatibility = 1.7
 
     repositories {
         maven { // The google mirror is less flaky than mavenCentral()