Lock loofah on Ruby < 2.5 on Rails <= 5.2

There's some incompatibility issue with loofah there since it uses an
older version of nokogiri, so I'm locking it on those older versions to
try to get a green build again there.

gemfiles/Gemfile-rails-4-1

@@ -44,3 +44,8 @@ if RUBY_VERSION < "2.3.0"
   # `Object.deprecate_constant` which isn't available.
   gem "multipart-post", "2.1.1"
 end
+
+if RUBY_VERSION < "2.5.0"
+  # There's a build incompatibility issue with nokogiri and loofah
+  gem "loofah", "~> 2.20.0"
+end

gemfiles/Gemfile-rails-4-2

@@ -43,3 +43,8 @@ if RUBY_VERSION < "2.3.0"
   # `Object.deprecate_constant` which isn't available.
   gem "multipart-post", "2.1.1"
 end
+
+if RUBY_VERSION < "2.5.0"
+  # There's a build incompatibility issue with nokogiri and loofah
+  gem "loofah", "~> 2.20.0"
+end

gemfiles/Gemfile-rails-5-0

@@ -35,3 +35,8 @@ if RUBY_VERSION < "2.3.0"
   # `Object.deprecate_constant` which isn't available.
   gem "multipart-post", "2.1.1"
 end
+
+if RUBY_VERSION < "2.5.0"
+  # There's a build incompatibility issue with nokogiri and loofah
+  gem "loofah", "~> 2.20.0"
+end

gemfiles/Gemfile-rails-5-1

@@ -28,3 +28,8 @@ if RUBY_VERSION < "2.3.0"
   # `Object.deprecate_constant` which isn't available.
   gem "multipart-post", "2.1.1"
 end
+
+if RUBY_VERSION < "2.5.0"
+  # There's a build incompatibility issue with nokogiri and loofah
+  gem "loofah", "~> 2.20.0"
+end

gemfiles/Gemfile-rails-5-2

@@ -22,3 +22,8 @@ end
 platforms :ruby do
   gem "sqlite3", "~> 1.3.6"
 end
+
+if RUBY_VERSION < "2.5.0"
+  # There's a build incompatibility issue with nokogiri and loofah
+  gem "loofah", "~> 2.20.0"
+end