Undefined check

Patch update to check if a message is undefined

Fluent support

Moved dependency from l20n to fluent (https://projectfluent.org).
Updated the get method to return a <React.Fragment> instead of a <span> tag.

React v16

React propTypes now use separate prop-types package.
This will remove the console warning when using React v16 and makes sure it won't break in future versions.