Don't include assertions in build

[priecint]

assertions are only used in augur tests so there is no need to send them to browser (i.e. bundle them with UI components).

Instead of
import { assertions } from 'augur-ui-react-components';
we can have something like
import assertions from 'augur-ui-react-components/lib/assertions';

[priecint]

I think the correct way how to deal with this is:
1/ add build:assertions script to package.json which builds src/assertions to lib/assertions - this would be later used in augur tests
2/ remove assertions from components.jsx

[tinybike]

Bumped to top priority (see AugurProject/augur#714 for explanation).

[tinybike]

Removing this from MVP as it does not solve the npm link problem.