Use environment variable instead of a hard-coded API_KEY

[rbval]

API_KEY is hardcoded and should not be in version control: https://github.com/StephenGrider/ReduxCasts/blob/master/video_browser/src/index.js#L8 .

dotenv-webpack (https://github.com/mrsteele/dotenv-webpack) could be used to load the API_KEY from a local .env file.