[librsvg] CI: Fix rustfmt job

[Jordan Petridis <jpetridis src gnome org>]

commit 7b9ee211583add2d433f3ebeb70f77849f47b0ae
Author: Jordan Petridis <jordanpetridis protonmail com>
Date:   Sun May 20 10:00:47 2018 +0000

    CI: Fix rustfmt job

 .gitlab-ci.yml | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index e219d654..03c2fa48 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -123,7 +123,8 @@ rustfmt:
   stage: lint
   script:
     - rustc --version && cargo --version
-    - cargo fmt --all -- --write-mode=diff
+    - cargo fmt --version
+    - cargo fmt --all -- --check
 
 
 # Configure and run clippy on nightly