added gem deep_merge to gemspec

[firedev]

When used in Rails together with Deep Merge gem Rails config produces the following error:

/[...]/gems/deep_merge-1.0.1/lib/deep_merge/core.rb:5: warning: already initialized constant DeepMerge::DEFAULT_FIELD_KNOCKOUT_PREFIX
/[...]/gems/rails_config-0.4.2/lib/rails_config/vendor/deep_merge.rb:4: warning: previous definition of DEFAULT_FIELD_KNOCKOUT_PREFIX was here

It is caused by special rails_compat module in deep_merge:

gem 'deep_merge', require: 'deep_merge/rails_compat'

That defines 'deeper_merge' methods on hash: https://github.com/danielsdeleo/deep_merge#using-deep_merge-in-rails

This pull request adds gem deep_merge as dependency to solve this once and for all.

[fredwu]

This is great, thank you @firedev! :)

[firedev]

Thanks, please bump the version up so I can use it in production without referring to GitHub.