Update Rust edition in Gotham tests

frameworks/Rust/gotham/Cargo.toml

@@ -2,6 +2,7 @@
 name = "gotham_techempower"
 version = "0.1.0"
 authors = ["Isaac Whitfield <[email protected]>"]
+edition = "2018"
 
 [dependencies]
 gotham = "0.4"