Documentation dependency on Nokogiri is not picked up by Bundler

:information_desk_person: The Nokogiri dependency in this project's Gemfile does not appear to be picked up by Bundler when running the documentation app, causing a LoadError exception to be raised. Moving the dependency into the gemspec as a runtime dependency resolves this issue.

I've added some more information around this issue below (Ruby version 2.1.1, Bundler version 1.6.2, Interpol version 0.10.6).

My Gemfile:
```ruby
source "https://rubygems.org"

gem "interpol", "0.10.6"
gem "sinatra"
```
```
$ bundle install
Resolving dependencies...
Using json-schema 1.1.1
Using rack 1.5.2
Using interpol 0.10.6
Using rack-protection 1.5.3
Using tilt 1.4.1
Using sinatra 1.4.5
Using bundler 1.6.2
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
```
My example documentation app:
```ruby
# doc_app.ru 
require 'interpol/documentation_app'
require 'sinatra'

doc_app = Interpol::DocumentationApp.build

run doc_app
```
```
bundle exec rackup doc_app.ru 
/Users/sg/.gem/ruby/2.1.1/gems/interpol-0.10.6/lib/interpol/documentation.rb:1:in `require': cannot load such file -- nokogiri (LoadError)
	from /Users/sg/.gem/ruby/2.1.1/gems/interpol-0.10.6/lib/interpol/documentation.rb:1:in `<top (required)>'
	from /Users/sg/.gem/ruby/2.1.1/gems/interpol-0.10.6/lib/interpol/documentation_app.rb:2:in `require'
	from /Users/sg/.gem/ruby/2.1.1/gems/interpol-0.10.6/lib/interpol/documentation_app.rb:2:in `<top (required)>'
	from /Users/sg/src/presentations/Twilio/working-with-apis-for-fun-and-profit/doc_app.ru:1:in `require'
	from /Users/sg/src/presentations/Twilio/working-with-apis-for-fun-and-profit/doc_app.ru:1:in `block in <main>'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/builder.rb:55:in `instance_eval'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/builder.rb:55:in `initialize'
	from /Users/sg/src/presentations/Twilio/working-with-apis-for-fun-and-profit/doc_app.ru:in `new'
	from /Users/sg/src/presentations/Twilio/working-with-apis-for-fun-and-profit/doc_app.ru:in `<main>'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/builder.rb:49:in `eval'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/builder.rb:49:in `new_from_string'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/builder.rb:40:in `parse_file'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/server.rb:277:in `build_app_and_options_from_config'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/server.rb:199:in `app'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/server.rb:314:in `wrapped_app'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/server.rb:250:in `start'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/lib/rack/server.rb:141:in `start'
	from /usr/local/rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/rack-1.5.2/bin/rackup:4:in `<top (required)>'
	from /Users/sg/.rbenv/versions/ruby-2.1.1/bin/rackup:23:in `load'
	from /Users/sg/.rbenv/versions/ruby-2.1.1/bin/rackup:23:in `<main>'
```