Set sitemap: false in defaults

This change tests whether the fix in jekyll/jekyll#6162 actually
addresses the problem described in jekyll/jekyll-sitemap#180, and finds
that it does _not_ fix the problem.

The problem is that binary files (pdf files in our case) do not inherit
`sitemap: false` like other Jekyll page types do.

The test scenario was simple:

 1. Upgrade to Jekyll 3.5.1, the first version that contains the changes
 in jekyll/jekyll#6162.

 2. Prior to the _config.yml changes in this commit, generate
 _site/sitemap.xml by running `bundle exec jekyll build`. This produces
 the expected result of a complete sitemap.xml wherein all pages are
 included by default.

 3. Copy _site/sitemap.xml => sitemap-before.xml

 4. Apply the changes in this commit and run `bundle exec jekyll build`
 once again. The expected result is an empty sitemap.xml, but the actual
 result is a sitemap.xml that contains no entries except for binary
 files.

 4. Copy _site/sitemap.xml => sitemap-after.xml

 5. `sdiff -w 120 sitemap-before.xml sitemap-after.xml` and paste the
 results into a comment on jekyll/jekyll-sitemap#180.

_config.yml

@@ -7,6 +7,11 @@ plugins:
   - jekyll-redirect-from
   - jekyll-feed
   - jekyll-sitemap
+defaults:
+  - scope:
+      path: ""
+    values:
+      sitemap: false
 feed:
   path: blog/feed.atom
 include: