Add extension onto flexboxgrid import

[nathanstitt]

Found one more thing 😁

This makes it a bit easier to mock when testing with Jest, since testing setups have a role saying to mock out '*.css'

I just lost a bit of time debugging why Jest wasn't mocking the css even though I had the recommended rule:

    "moduleNameMapper": {
        "\\.(css|less|scss)$": "<rootDir>/config/styleMock.js"
    }

For now I've changed it to:

    "moduleNameMapper": {
        "\\.(css|less|scss)$": "<rootDir>/config/styleMock.js",
        "flexboxgrid$": "<rootDir>/config/styleMock.js"
   }

which works fine, but would be nicer if I could just use the original rule, especially since the Jest Docs recommend that.

[silvenon]

Nice!