mime-types is a runtime dependency

Since [mime-types](https://github.com/uploadcare/ruby-uploadcare-api/blob/master/lib/uploadcare/uploader.rb#L1) is a runtime dependency, noting it as such in the gemspec in order to avoid an error:

Gemfile:

```ruby
source 'https://rubygems.org'
gem 'uploadcare-api'
```

Command line:

```bash
$>  bundle
Fetching gem metadata from https://rubygems.org/..........
Fetching gem metadata from https://rubygems.org/..
Resolving dependencies...
Using multipart-post (1.2.0)
Using faraday (0.8.8)
Installing uploadcare-api (0.1.3)
Using bundler (1.3.5)
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
$>  ruby test.rb
/Users/brettb/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/uploadcare-api-0.1.3/lib/uploadcare/uploader.rb:1:in `require': cannot load such file -- mime/types (LoadError)
	from /Users/brettb/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/uploadcare-api-0.1.3/lib/uploadcare/uploader.rb:1:in `<top (required)>'
	from /Users/brettb/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/uploadcare-api-0.1.3/lib/uploadcare.rb:6:in `require'
	from /Users/brettb/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/uploadcare-api-0.1.3/lib/uploadcare.rb:6:in `<top (required)>'
	from test.rb:4:in `require'
	from test.rb:4:in `<main>'
```