Make the i18n gem a normal dependency instead of just a development dependency

I just installed `phrase` on a relatively clean system. The installation ran smoothly:

```bash
$ sudo gem install phrase
[sudo] password for h2s: 
Fetching: colorize-0.5.8.gem (100%)
Successfully installed colorize-0.5.8
Fetching: phrase-0.4.7.gem (100%)
Successfully installed phrase-0.4.7
Parsing documentation for colorize-0.5.8
Installing ri documentation for colorize-0.5.8
Parsing documentation for phrase-0.4.7
Installing ri documentation for phrase-0.4.7
Done installing documentation for colorize, phrase (1 sec).
2 gems installed
```

But `phrase` didn't actually work when it was done:

```bash
$ phrase
/usr/local/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require': cannot load such file -- i18n (LoadError)
        from /usr/local/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
        from /usr/local/lib/ruby/gems/2.0.0/gems/phrase-0.4.7/lib/phrase/engine.rb:4:in `<top (required)>'
        from /usr/local/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
        from /usr/local/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
        from /usr/local/lib/ruby/gems/2.0.0/gems/phrase-0.4.7/lib/phrase.rb:54:in `<module:Phrase>'
        from /usr/local/lib/ruby/gems/2.0.0/gems/phrase-0.4.7/lib/phrase.rb:6:in `<top (required)>'
        from /usr/local/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
        from /usr/local/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
        from /usr/local/lib/ruby/gems/2.0.0/gems/phrase-0.4.7/lib/phrase/tool.rb:5:in `<top (required)>'
        from /usr/local/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
        from /usr/local/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
        from /usr/local/lib/ruby/gems/2.0.0/gems/phrase-0.4.7/bin/phrase:3:in `<top (required)>'
        from /usr/local/bin/phrase:23:in `load'
        from /usr/local/bin/phrase:23:in `<main>'
```

The error message seemed to indicate that `phrase` was trying to load the `i18n` gem, which wasn't present on my system. So I installed that gem, and then `phrase` seemed to work just fine:

```bash
$ sudo gem install i18n
Fetching: i18n-0.6.4.gem (100%)
Successfully installed i18n-0.6.4
Parsing documentation for i18n-0.6.4
Installing ri documentation for i18n-0.6.4
Done installing documentation for i18n (1 sec).
1 gem installed
$ phrase
usage: phrase <command> [<args>]

  phrase init --secret=<YOUR SECRET> --default-locale=<DEFAULT LOCALE, e.g. en> --default-format=<FORMAT, e.g. yml> --default-target=<TARGET, default is ./phrase/locales/>

  phrase push FILE [--tags=<tags>] [--locale=<locale>]
  phrase push DIRECTORY [--tags=<tags>] [--locale=<locale>]

  phrase pull [LOCALE] [--target=<target-folder>] [--format=<format>] [--tag=<tag>]

  phrase tags [-l, --list]

  phrase --version
```

It looks like `i18n` is a full-blown dependency of `phrase` now, so this PR makes a tiny tweak to the gemspec to reflect that fact.