Specify rails gem for pre rails master

https://ci.appveyor.com/project/bf4/active-model-serializers/build/1144/job/rf3yi4b7vf8w4t6l

```
Build started
git clone -q --branch=fix_ci https://github.com/rails-api/active_model_serializers.git C:\projects\active-model-serializers
git checkout -qf 37d4bdc4d2
Running Install scripts
SET PATH=C:\Ruby%ruby_version%\bin;%PATH%
ruby --version
ruby 2.0.0p648 (2015-12-16) [i386-mingw32]
gem --version
2.6.13
gem install bundler
Successfully installed bundler-1.16.0
Parsing documentation for bundler-1.16.0
Installing ri documentation for bundler-1.16.0
1 gem installed
bundler --version
Bundler version 1.16.0
bundle platform
Your platform is: i386-mingw32
Your app has gems that work on these platforms:
* x86-mingw32
Your Gemfile does not specify a Ruby version requirement.
bundle install --path=vendor/bundle --retry=3 --jobs=3
Fetching gem metadata from https://rubygems.org/..........
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...........................................
Bundler could not find compatible versions for gem "actionpack":
  In Gemfile:
    actionpack (~> 4.0.0) x86-mingw32
Could not find gem 'actionpack (~> 4.0.0)' in any of the sources.
Bundler could not find compatible versions for gem "activemodel":
  In Gemfile:
    activemodel (~> 4.0.0) x86-mingw32
    active_model_serializers x86-mingw32 was resolved to 0.8.4, which depends on
      activemodel (>= 3.0) x86-mingw32
    activerecord (~> 4.0.0) x86-mingw32 was resolved to 4.0.0, which depends on
      activemodel (= 4.0.0) x86-mingw32
Bundler could not find compatible versions for gem "activesupport":
  In Gemfile:
    activesupport (~> 4.0.0) x86-mingw32
    actionpack (~> 4.0.0) x86-mingw32 was resolved to 4.0.0, which depends on
      activesupport (= 4.0.0) x86-mingw32
Bundler could not find compatible versions for gem "ruby":
  In Gemfile:
    ruby x86-mingw32
    pry x86-mingw32 was resolved to 0.11.2, which depends on
      ruby (>= 1.9.3) x86-mingw32
    rails (>= 3.0) x86-mingw32 was resolved to 5.1.4, which depends on
      ruby (>= 2.2.2) x86-mingw32
Bundler could not find compatible versions for gem "tzinfo":
  In Gemfile:
    actionpack (~> 4.0.0) x86-mingw32 was resolved to 4.0.0, which depends on
activesupport (= 4.0.0) x86-mingw32 was resolved to 4.0.0, which depends
on
        tzinfo (~> 0.3.37) x86-mingw32
    tzinfo-data x86-mingw32 was resolved to 1.2017.3, which depends on
      tzinfo (>= 1.0.0) x86-mingw32
Command exited with code 6
```

Gemfile

@@ -17,6 +17,7 @@ if version == 'master'
   end
 else
   gem_version = "~> #{version}.0"
+  gem 'rails', gem_version
   gem 'railties', gem_version
   gem 'activesupport', gem_version
   gem 'activemodel', gem_version