travis: container based builds have been deprecated (#34642)

We were not using sudo, but now the main images should be similarly fast. More at:
https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

- [x] I have read [freeCodeCamp's contribution guidelines](https://github.com/freeCodeCamp/freeCodeCamp/blob/master/CONTRIBUTING.md).
- [x] My pull request has a descriptive title (not a vague title like `Update index.md`)
- [x] My pull request targets the `master` branch of freeCodeCamp.
- [x] None of my changes are plagiarized from another source without proper attribution.
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.travis.yml

@@ -12,8 +12,6 @@ env:
     - NO_UPDATE_NOTIFIER=1
     - NODE_NO_WARNINGS=1
 
-sudo: false
-
 before_install:
   - npm config set loglevel warn
 

curriculum/.travis.yml

@@ -27,5 +27,3 @@ deploy:
 branches:
   except:
     - /^v\d+\.\d+\.\d+$/
-
-sudo: false