Ruby 2.0 compatibility issues

[hakoerber]

I'm running CentOS7 from the offical centos:7 docker image, and am trying to install dotledger.
CentOS7 has the following ruby version:

# ruby --version
ruby 2.0.0p648 (2015-12-16) [x86_64-linux]
# bundle --version
Bundler version 1.7.8

When running bundle install, I get the following error:

Gem::InstallError: active_model_serializers requires Ruby version >= 2.1.
An error occurred while installing active_model_serializers (0.10.3), and Bundler cannot continue.
Make sure that `gem install active_model_serializers -v '0.10.3'` succeeds before bundling.

After installing ruby 2.1 (through rvm), the above commands succeeds. Ruby version:

# ruby --version
ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-linux]
# bundler --version
Bundler version 1.13.7

I have no experience with ruby gem install, and cannot contribute a solution except the ruby upgrade, but I think that we have two possibilities here:

• Use a active_model_serializers version compatible to ruby 2.0
• Require ruby 2.1 in the README (right now it requires Ruby (>= 2.0))

I'd send a pull request for the second solution if you want.