[howto/build-env] Add Ruby version.
[howto/build-env] Add Ruby version.

Summary: Ruby 2.3.0 generates different Podfile.lock output than Ruby 2.0.0, so documenting our team's use of Ruby 2.0.0 until we all move forward.

Reviewers: ajsecord, #mdc_ios_owners

Reviewed By: ajsecord, #mdc_ios_owners

Subscribers: ajsecord

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D730

randallli/material-components-ios,Eccelor/material-components-ios,sigito/material-components-ios,sigito/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,Eccelor/material-components-ios,samnm/material-components-ios,samnm/material-components-ios,chriscox/material-components-ios,sigito/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,samnm/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,willlarche/material-components-ios,willlarche/material-components-ios,willlarche/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,material-components/material-components-ios,Eccelor/material-components-ios,randallli/material-components-ios,chriscox/material-components-ios,Eccelor/material-components-ios,samnm/material-components-ios,brianjmoore/material-components-ios,brianjmoore/material-components-ios,chriscox/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,sigito/material-components-ios,brianjmoore/material-components-ios