Merge branch 'fix-ci-missing-xargo' into 'master'

Update .gitlab-ci.yml

See merge request redox-os/redox!1248

.gitlab-ci.yml

@@ -33,8 +33,8 @@ img:
      - |
         source "$HOME/.cargo/env" &&
         cargo install --version 0.1.1 cargo-config &&
-        cargo install --version 0.3.20 cargo-xbuild &&
-        cargo install xargo &&
+        cargo install cargo-xbuild &&
+        cargo install --version 0.3.20 xargo &&
         make ci-img IMG_TAG=$CI_COMMIT_REF_NAME
     artifacts:
         paths: