revert upgrade of jekyll-redirect-from

because CI complained:

> jekyll-redirect-from-0.16.0 requires ruby version >= 2.4.0, which is
> incompatible with the current version, ruby 2.3.1p112

we should move to Ruby 2.4 eventually I guess, but it doesn't
need to be now

Author: SethTisue

Gemfile.lock

@@ -37,7 +37,7 @@ GEM
       pathutil (~> 0.9)
       rouge (>= 1.7, < 4)
       safe_yaml (~> 1.0)
-    jekyll-redirect-from (0.16.0)
+    jekyll-redirect-from (0.15.0)
       jekyll (>= 3.3, < 5.0)
     jekyll-sass-converter (1.5.2)
       sass (~> 3.4)