setting more specific range for dependency of 'mime-type' gem

this fixes bundler yelling about dependencies when running `bundle install` for
`hammer-cli-katello`

```
$ bundle update
Fetching gem metadata from https://rubygems.org/........
Fetching gem metadata from https://rubygems.org/..
Resolving dependencies...
Bundler could not find compatible versions for gem "mime-types":
  In Gemfile:
    hammer_cli (>= 0) ruby depends on
      mime-types (< 2.0.0) ruby

    hammer_cli_katello (>= 0) ruby depends on
      mime-types (2.0)
```