Added /email/ route to robots.txt

closes https://github.com/TryGhost/Team/issues/952

- The `/email/` route will be a home for email only posts. We are adding the route preemptively to have the crowlers update their caches before the feature sees the light of The Internet

core/server/public/robots.txt

@@ -2,3 +2,4 @@ User-agent: *
 Sitemap: {{blog-url}}/sitemap.xml
 Disallow: /ghost/
 Disallow: /p/
+Disallow: /email/

test/frontend-acceptance/default_routes.test.js

@@ -300,7 +300,8 @@ describe('Default Frontend routing', function () {
             res.text.should.equal(
                 'User-agent: *\n' +
                 'Sitemap: http://127.0.0.1:2369/sitemap.xml\nDisallow: /ghost/\n' +
-                'Disallow: /p/\n'
+                'Disallow: /p/\n' +
+                'Disallow: /email/\n'
             );
         });