work around for TypeMismatch error

Work around a TypeMismatchError with Ruby 2.0 and
ActiveSupport 4.0.0 when only requiring core_ext:

/opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:106:in `require': superclass mismatch for class DateTime (TypeError)
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:106:in `require'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/date.rb:3:in `<top (required)>'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:106:in `require'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:106:in `require'
	from /home/jk/.gem/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/core_ext/date/calculations.rb:1:in `<top (required)>'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:58:in `require'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:58:in `require'
	from /home/jk/.gem/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/core_ext/date.rb:2:in `<top (required)>'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:106:in `require'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:106:in `require'
	from /home/jk/.gem/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/core_ext.rb:3:in `block in <top (required)>'
	from /home/jk/.gem/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/core_ext.rb:1:in `each'
	from /home/jk/.gem/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/core_ext.rb:1:in `<top (required)>'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:58:in `require'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:58:in `require'
	from /home/jk/.gem/ruby/2.0.0/gems/earthquake-1.0.0/lib/earthquake.rb:14:in `block in <top (required)>'
	from /home/jk/.gem/ruby/2.0.0/gems/earthquake-1.0.0/lib/earthquake.rb:14:in `each'
	from /home/jk/.gem/ruby/2.0.0/gems/earthquake-1.0.0/lib/earthquake.rb:14:in `<top (required)>'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:58:in `require'
	from /opt/rubies/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:58:in `require'
	from /home/jk/.gem/ruby/2.0.0/gems/earthquake-1.0.0/bin/earthquake:34:in `<top (required)>'
	from /home/jk/.gem/ruby/2.0.0/bin/earthquake:23:in `load'
	from /home/jk/.gem/ruby/2.0.0/bin/earthquake:23:in `<main>'