Remove railties runtime dependency

There's no need to specify a dependency on railties for use in a Rails app. I'm trying to use jira-ruby outside of Rails (with Dashing) and I'm running into dependency issues because of railties:

```
Bundler could not find compatible versions for gem "sprockets":
  In Gemfile:
    railties (>= 0) ruby depends on
      sprockets (~> 2.1.3) ruby

    dashing (~> 1.0) ruby depends on
      sprockets (2.8.1)

Bundler could not find compatible versions for gem "thor":
  In Gemfile:
    railties (>= 0) ruby depends on
      thor (~> 0.14.6) ruby

    dashing (~> 1.0) ruby depends on
      thor (0.16.0)
```