chore: adjust .travis to install latest npm (#812)

As recommended by @ljharb https://github.com/testing-library/react-testing-library/pull/809/files#r514441103

Author: kentcdodds

.travis.yml

@@ -12,10 +12,9 @@ env:
   - REACT_DIST=latest
   - REACT_DIST=next
   - REACT_DIST=experimental
+before_install:
+  - nvm install-latest-npm
 install:
-  # 7.0.6 fixed CI environments prompting user input.
-  # Can be removed once node15 bumps the shipped npm version.
-  - npm install --global npm@>=7.0.6
   - npm install
   # as requested by the React team :)
   # https://reactjs.org/blog/2019/10/22/react-release-channels.html#using-the-next-channel-for-integration-testing