Add a Gemfile to pin Ruby dependencies

Otherwise breaking changes in e.g. jekyll interface will render the
development environment broken.

Gemfile

@@ -0,0 +1,7 @@
+source 'https://rubygems.org'
+
+gem 'jekyll', '~> 3.4.0'
+gem 'jekyll-sitemap'
+gem 'jekyll-redirect-from'
+gem 'jekyll-feed'
+gem 'foreman'

Gemfile.lock

@@ -0,0 +1,60 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    addressable (2.5.1)
+      public_suffix (~> 2.0, >= 2.0.2)
+    colorator (1.1.0)
+    ffi (1.9.18)
+    foreman (0.84.0)
+      thor (~> 0.19.1)
+    forwardable-extended (2.6.0)
+    jekyll (3.4.4)
+      addressable (~> 2.4)
+      colorator (~> 1.0)
+      jekyll-sass-converter (~> 1.0)
+      jekyll-watch (~> 1.1)
+      kramdown (~> 1.3)
+      liquid (~> 3.0)
+      mercenary (~> 0.3.3)
+      pathutil (~> 0.9)
+      rouge (~> 1.7)
+      safe_yaml (~> 1.0)
+    jekyll-feed (0.9.2)
+      jekyll (~> 3.3)
+    jekyll-redirect-from (0.12.1)
+      jekyll (~> 3.3)
+    jekyll-sass-converter (1.5.0)
+      sass (~> 3.4)
+    jekyll-sitemap (1.1.1)
+      jekyll (~> 3.3)
+    jekyll-watch (1.5.0)
+      listen (~> 3.0, < 3.1)
+    kramdown (1.13.2)
+    liquid (3.0.6)
+    listen (3.0.8)
+      rb-fsevent (~> 0.9, >= 0.9.4)
+      rb-inotify (~> 0.9, >= 0.9.7)
+    mercenary (0.3.6)
+    pathutil (0.14.0)
+      forwardable-extended (~> 2.6)
+    public_suffix (2.0.5)
+    rb-fsevent (0.9.8)
+    rb-inotify (0.9.10)
+      ffi (>= 0.5.0, < 2)
+    rouge (1.11.1)
+    safe_yaml (1.0.4)
+    sass (3.4.24)
+    thor (0.19.4)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  foreman
+  jekyll (~> 3.4.0)
+  jekyll-feed
+  jekyll-redirect-from
+  jekyll-sitemap
+
+BUNDLED WITH
+   1.15.1