Yarn not installing packages

I'm trying to downgrade my version of ember-cli-code-coverage and once I change the version in my package.json and run yarn install it "installs" but when I go to run the coverage test COVERAGE=true ember test it gives me this error.

Missing yarn packages: 
Package: ember-cli-code-coverage
  * Specified: v1.0.0-beta.6
  * Installed: 1.0.0-beta.8

Run `yarn` to install missing dependencies.

So run yarn and then this is what I get.

yarn install v1.3.2
(node:8156) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
[1/5] Validating package.json...
[2/5] Resolving packages...
success Already up-to-date.
Done in 1.15s.

package.json (under devDependencies): "ember-cli-code-coverage": "v1.0.0-beta.5"