Resume testing against AR 4.0

There are obviously people still using it, unfortunately. They should
update immediately, as AR < 4.2 is EOL, and PT will drop support very
soon.

.travis.yml

@@ -24,6 +24,7 @@ before_script:
   - sh -c "if [ \"$DB\" = 'postgres' ]; then psql -c 'create database paper_trail_foo;'  -U postgres; fi"
 
 gemfile:
+  - gemfiles/ar_4.0.gemfile
   - gemfiles/ar_4.2.gemfile
   - gemfiles/ar_5.0.gemfile
   - gemfiles/ar_master.gemfile

Appraisals

@@ -7,6 +7,11 @@
 # > the version from the appraisal takes precedence.
 # > https://github.com/thoughtbot/appraisal
 
+appraise "ar-4.0" do
+  gem "activerecord", "~> 4.0"
+  gem "sinatra", "~> 1.4.6"
+end
+
 appraise "ar-4.2" do
   gem "activerecord", "~> 4.2"
   gem "sinatra", "~> 1.4.6"

gemfiles/ar_4.0.gemfile

@@ -0,0 +1,8 @@
+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "activerecord", "~> 4.0"
+gem "sinatra", "~> 1.4.6"
+
+gemspec :path => "../"