Don't disable invalid login form.

Author: oampo

src/components/login-form.js

@@ -41,7 +41,7 @@ export class LoginForm extends React.Component {
                     id="password"
                     validate={[required, nonEmpty]}
                 />
-                <button disabled={this.props.invalid || this.props.submitting}>
+                <button disabled={this.props.pristine || this.props.submitting}>
                     Log in
                 </button>
             </form>