Exclude yargs from david checks

Yargs 8 requires Node 4.  Until this package drops support for Node
0.12, avoid requiring yargs 8.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

Author: kevinoid

package.json

@@ -31,7 +31,7 @@
     "lint-js": "node-version-gte-4 && eslint . && echo ESLint passed. || node-version-lt-4",
     "postpublish": "git -C doc push && git push --follow-tags origin master gh-pages && echo Remember to update GitHub Releases from CHANGELOG.md && echo until skywinder/github-changelog-generator#56 is fixed.",
     "postversion": "rimraf doc && git clone -b gh-pages -l -q . doc && npm run doc && git -C doc add . && git -C doc commit -n -m \"Docs for v$npm_package_version\"",
-    "preversion": "depcheck --ignores eslint-plugin-import --ignore-dirs doc && david -i nock && git-branch-is master && travis-status -b master -c -wx && appveyor-status -b master -c -w -p kevinoid/swagger-spec-validator && istanbul check-coverage --statements 95 coverage/coverage.json",
+    "preversion": "depcheck --ignores eslint-plugin-import --ignore-dirs doc && david -i nock,yargs && git-branch-is master && travis-status -b master -c -wx && appveyor-status -b master -c -w -p kevinoid/swagger-spec-validator && istanbul check-coverage --statements 95 coverage/coverage.json",
     "test": "npm run lint && npm run test-unit",
     "test-cov": "npm run lint && npm run test-unit-cov",
     "test-unit": "mocha --recursive test",