When Capybara/Cucumber tests fail:
```ruby
undefined method `+' for nil:NilClass (NoMethodError)
.../.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/minitest-5.10.1/lib/minitest/assertions.rb:135:in `assert'
```
Do this:
1. Install test-unit
```ruby
gem 'test-unit'
```
2. Include assertions in env.rb
```ruby
World Test::Unit::Assertions
```