Add explicit install_requires for BeautifulSoup4 and SoupSieve

Since we import both, I think we should list them in install_requires.

I don't know if we should pin BeautifulSoup4 to >= 4.7.0, which is the
version where BeautifulSoup4 started depending on SoupSieve.  If we
don't, browser.getLink(id='foo.bar') might not work correctly?

setup.py

@@ -65,6 +65,8 @@
         'zope.cachedescriptors',
         'pytz > dev',
         'WebTest >= 2.0.30',
+        'BeautifulSoup4', # do we need to require >= 4.7.0?
+        'SoupSieve >= 1.9.0',
         'WSGIProxy2',
         'six',
     ],