Expand build matrix

Moving our apps to Ruby 2.4 and Rails 5 will take some time. Until that
is complete we will have to support several different combinations of
versions. This expands the build matrix to help ensure this coverage.

Author: cupakromer

.travis.yml

@@ -4,4 +4,13 @@ cache: bundler
 sudo: false
 script: bin/rake
 rvm:
-  - 2.3.1
+  - 2.3.3
+  - 2.4.0
+env:
+  - RAILS_VERSION='~> 5.0.0'
+  - RAILS_VERSION='~> 4.2.0'
+matrix:
+  include:
+    - rvm: 2.2.3
+      env: RAILS_VERSION="4.2.5"
+  fast_finish: true

Gemfile

@@ -12,3 +12,6 @@ gemspec
 
 # To use debugger
 # gem 'debugger'
+
+rails_dependencies_gemfile = File.expand_path("../Gemfile-rails-dependencies", __FILE__)
+eval_gemfile rails_dependencies_gemfile

Gemfile-rails-dependencies

@@ -0,0 +1,12 @@
+version_file = File.expand_path("../.rails-version", __FILE__)
+version = ENV.fetch('RAILS_VERSION') {
+  File.read(version_file).chomp if File.exist?(version_file)
+}
+
+if version && !version.empty?
+  gem "rails", version
+elsif RUBY_VERSION.to_f < 2.4
+  gem "rails", "~> 4.2.0"
+else
+  gem "rails", "~> 5.0"
+end