Fix tumblr_client

tumblr_client isn't currently compatible with Faraday 0.9.0, may users are seeing this error:

```
ERROR: undefined method `client=' for #<Faraday::ConnectionOptions:0x007f8968bfbaf0>
/Users/davidgolightly/.rvm/gems/ruby-2.1.2@distiller/gems/faraday-0.9.0/lib/faraday/options.rb:31:in `block in update'
/Users/davidgolightly/.rvm/gems/ruby-2.1.2@distiller/gems/faraday-0.9.0/lib/faraday/options.rb:20:in `each'
/Users/davidgolightly/.rvm/gems/ruby-2.1.2@distiller/gems/faraday-0.9.0/lib/faraday/options.rb:20:in `update'
/Users/davidgolightly/.rvm/gems/ruby-2.1.2@distiller/gems/faraday-0.9.0/lib/faraday/options.rb:52:in `merge'
/Users/davidgolightly/.rvm/gems/ruby-2.1.2@distiller/gems/faraday-0.9.0/lib/faraday.rb:69:in `new'
/Users/davidgolightly/.rvm/gems/ruby-2.1.2@distiller/gems/tumblr_client-0.8.2/lib/tumblr/connection.rb:19:in `connection'
/Users/davidgolightly/.rvm/gems/ruby-2.1.2@distiller/gems/tumblr_client-0.8.2/lib/tumblr/request.rb:8:in `get_response'
/Users/davidgolightly/.rvm/gems/ruby-2.1.2@distiller/gems/tumblr_client-0.8.2/lib/tumblr/request.rb:26:in `get'
/Users/davidgolightly/.rvm/gems/ruby-2.1.2@distiller/gems/tumblr_client-0.8.2/lib/tumblr/blog.rb:40:in `posts'
```

Reverting the dependency to 0.8.9 fixes this error.

Note: I am not, nor am I affiliated with, phoenixwizard. I just noticed a ton of forks of this repo doing essentially the same thing, and rather than create my own fork, I thought I'd propose this fix for you guys.