Fix #1684: Use the correct flag name in rustc linter

The rust compiler renamed the option '-Z no-trans' to '-Z no-codegen'.

https://github.com/rust-lang/rust-enhanced/issues/281

ale_linters/rust/rustc.vim

@@ -1,7 +1,7 @@
 " Author: Daniel Schemala <istjanichtzufassen@gmail.com>
 " Description: rustc for rust files
 
-call ale#Set('rust_rustc_options', '-Z no-trans')
+call ale#Set('rust_rustc_options', '-Z no-codegen')
 
 function! ale_linters#rust#rustc#RustcCommand(buffer) abort
     " Try to guess the library search path. If the project is managed by cargo,