Remove Ruby 2.1 support

[Ana06]

I built and installed trollolo with Ruby 2.1 and it is not working:

Ana@linux-nt5w:~/Documents/trollolo(master)> gem.ruby2.1 build trollolo.gemspec 
WARNING:  pessimistic dependency on ruby-trello (~> 1.5.0) may be overly strict
  if ruby-trello is semantically versioned, use:
    add_runtime_dependency 'ruby-trello', '~> 1.5', '>= 1.5.0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for help
  Successfully built RubyGem
  Name: trollolo
  Version: 0.0.11
  File: trollolo-0.0.11.gem

Ana@linux-nt5w:~/Documents/agile-team-dashboard(burndown-chart)> vagrant exec sudo gem.ruby2.1 install trollolo
Building native extensions.  This could take a while...
ERROR:  Error installing trollolo:
        ERROR: Failed to build gem native extension.

    current directory: /usr/lib64/ruby/gems/2.1.0/gems/unf_ext-0.0.7.4/ext/unf_ext
/usr/bin/ruby.ruby2.1 -r ./siteconf20170913-4441-1e8v44t.rb extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib64/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /usr/lib64/ruby/gems/2.1.0/gems/unf_ext-0.0.7.4 for inspection.
Results logged to /usr/lib64/ruby/gems/2.1.0/extensions/x86_64-linux/2.1.0/unf_ext-0.0.7.4/gem_make.out

I would suggest to remove Ruby 2.1 support as it is quite old. This way we can also remove activesupport dependency which seems to be introduced only for staying compatible with Ruby 2.1: 1b97fc5