Update spec broken due to Enzyme update

Enzyme incoroporated a breaking change, which still matched our version
specification for the npm package. The change caused our Travis build to
fail. This commit updates the spec accordingly.

See:

- enzymejs/enzyme#523
- enzymejs/enzyme#510 (comment)

spec/popup/components/header-spec.jsx

@@ -31,6 +31,6 @@ describe('header', () => {
 
   it('hides the summary button when no tickets are found', () => {
     const wrapper = shallow(<Header tickets={[]} />);
-    expect(wrapper.some(CopyButton)).toBe(false);
+    expect(wrapper.find(CopyButton).isEmpty()).toBe(true);
   });
 });