Auto merge of #7974 - frewsxcv:travis, r=jdm

Use Travis container-based infra for test-tidy

I wasn't aware one could specify options like `sudo` and `services` on a per build-environment basis. Since the container-based builds are faster than normal builds, we should utilize it for test-tidy

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.travis.yml

@@ -1,7 +1,4 @@
 language: python
-sudo: 9000
-services:
-  - docker
 
 before_install:
   - if [ "$DOCKER" = "1" ]; then docker build -t servo etc/ci/; fi
@@ -13,10 +10,14 @@ script:
 matrix:
   fast_finish: true
   include:
-    - env:
+    - sudo: false
+      env:
         - CMD="./mach test-tidy"
         - DOCKER=0
-    - env:
+    - sudo: 9000
+      services:
+        - docker
+      env:
         - CMD="./mach build -d --verbose"
         - DOCKER=1