Add minimum ruby version requirement

This only specifies "1.9" rather than "1.9.3" since it appears 
that JRuby in 1.9 mode reports its version as "1.9", without 
the third version number segment.
Merge pull request #2589 from jordoh/master

Add required ruby version to gemspec
Allow secondary usage on m/r with inline output. [ fix #2584 ]
Extra guard against nil session override. [ #2583 ]
Fix issue number [ #2582 ]
Perform reloads with strong consistency
Update changelog
Ensure session config passes symbols to Moped.

[ fix #2585 ]
Fix method clash when resetting unloaded criteria.

[ fix #2587 ]
Block calls to #define_dynamic_writer with invalid method names
Update changelog
made attributes class same as attrs
Update changelog
Fix localized field sorting on embedded docs.

[ fix #2598 ]
Fix cascade when metadata exists w/o strategy.

[ fix #2630 ]
Update perf/profile.rb

ensure GB.enable
Return proper observed_classes when setting models via observe.

[ fix #2619 ]
Expose klass in DocumentNotFound errors.

[ fix #2616 ]
Ensure that calling first after last with sorting resets.

[ fix #2610 ]
Hand through batch_size option.