[rust][gn] Specify Rust 2015

Change-Id: Ib9febe075f55638e27aa2435984f8f5548eb2a12
diff --git a/BUILD.gn b/BUILD.gn
index 8cf5d97..7f4765d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -10,6 +10,7 @@
 
 rustc_library("rustyline") {
   name = "rustyline"
+  edition = "2015"
   deps = [
     "//garnet/public/rust/crates/fuchsia-device",
     "//garnet/public/rust/crates/fuchsia-zircon",