Added test to check for accidental extraneous dependencies

[lpalmes]

Fixes #1720
It just checks package.json and check if includes react, react-dom, react-scripts, if other is found it will fail the test.

[n3tr]

Hmm.. It is failing on CI, but works fine on my machine (OSX).

Looks like grep works a bit different on macOS and Linux

# macOS Sierra
cat package.json | grep -E '^\s*"react(?:-dom|-scripts)?"'
    "react": "^15.4.2",
    "react-dom": "^15.4.2"
    "react-scripts": "0.9.5"

# Ubuntu 16.04 (on vagrant)
cat package.json | grep -E '^\s*"react(?:-dom|-scripts)?"'
    "react": "^15.4.2",
    "react-scripts": "0.9.5"

[Timer]

Looks like grep doesn't support a non-capturing group on Ubuntu, we need to remove ?:.

Also, @lpalmes -- I don't think we need a whole new install to test this. We should be able to make it a function and test every install.

[Timer]

Thanks so much for this, @lpalmes!