Revert "Ignore codegen build failure associated with rustfmt bug"

Rustfmt bug has been fixed.

This reverts commit 19f8ce8ea429f2290de7020e49ba71e8e4cc7060.
diff --git a/.travis.yml b/.travis.yml
index d81b342..19b988d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -65,8 +65,6 @@
   allow_failures:
     - rust: nightly
       name: Clippy
-    - rust: nightly # rustfmt bug: https://github.com/rust-lang/rustfmt/issues/3575
-      name: Codegen
   fast_finish: true
 
 env: