Update CI configuration to use redoxer

.gitlab-ci.yml

-image: "redoxos/redox"
+image: "redoxos/redoxer"
 
 stages:
     - build
     - test
 
-before_script:
-    - rustup target add x86_64-unknown-redox --toolchain nightly
-    - rustup show      # Print version info for debugging
-
 cache:
     paths:
         - target/
 
-build:linux:cargo:
+build:linux:
     stage: build
     script: cargo +nightly build --verbose
 
-build:redox:cargo:
+build:redox:
     stage: build
-    script: 
-        - mkdir .cargo
-        - echo -e "[target.x86_64-unknown-redox]\nlinker = \"x86_64-unknown-redox-gcc\"" > .cargo/config
-        - cargo +nightly build --verbose --target x86_64-unknown-redox
+    script: redoxer build --verbose
 
-test:linux:cargo:
+test:linux:
     stage: test
-    dependencies: 
-        - build:linux:cargo
+    dependencies: build:linux
     script: cargo +nightly test --verbose
 
-# TODO: Set up a docker image with a redox vm that would allow to
-#   run things like tests under redox
+test:redox:
+    stage: test
+    dependencies: test:redox
+    script: redoxer test --verbose