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
@@ -13,12 +10,16 @@ script:
 matrix:
   fast_finish: true
   include:
-    - env:
+    - sudo: false
-      - CMD="./mach test-tidy"
+      env:
-      - DOCKER=0
+        - CMD="./mach test-tidy"
-    - env:
+        - DOCKER=0
-      - CMD="./mach build -d --verbose"
+    - sudo: 9000
-      - DOCKER=1
+      services:
+        - docker
+      env:
+        - CMD="./mach build -d --verbose"
+        - DOCKER=1
 
 branches:
   only: