Catch ExpectationNotMetError explicitly on RSpec 3

From RSpec 3 + Turnip,  the following differences began to appear.

**RSpec 2.14 + Turnip 1.2.2**:

```
$ bundle exec rspec examples/errors.feature:8
Run options: include {:locations=>{"./examples/errors.feature"=>[8]}}
F

Failures:

  1) raises errors Incorrect expectation there is a monster -> it should die
     Failure/Error: Then it should die
       
       expected: 0
            got: 1
       
       (compared using ==)
     # ./examples/steps/steps.rb:14:in `block in <top (required)>'
     # ./lib/turnip/execute.rb:20:in `step'
     # ./lib/turnip/rspec.rb:44:in `run_step'
     # ./examples/errors.feature:8:in `block (6 levels) in run'
     # ./examples/errors.feature:7:in `each'
     # ./examples/errors.feature:7:in `block (5 levels) in run'
     # ./examples/errors.feature:8:in `it should die'
```

**RSpec 3 + Turnip 1.2.2**:

```
$ bundle exec rspec examples/errors.feature:8
Run options: include {:locations=>{"./examples/errors.feature"=>[8]}}
F

Failures:

  1) raises errors Incorrect expectation there is a monster -> it should die
     Failure/Error: Then it should die
       
       expected: 0
            got: 1
       
       (compared using ==)
     # ./examples/steps/steps.rb:14:in `block in <top (required)>'
     # ./lib/turnip/execute.rb:20:in `step'
     # ./lib/turnip/rspec.rb:44:in `run_step'
     # ./examples/errors.feature:8:in `block (6 levels) in run'
     # ./examples/errors.feature:7:in `each'
     # ./examples/errors.feature:7:in `block (5 levels) in run'
```

diff:

```diff
      # ./examples/errors.feature:8:in `block (6 levels) in run'
      # ./examples/errors.feature:7:in `each'
      # ./examples/errors.feature:7:in `block (5 levels) in run'
-     # ./examples/errors.feature:8:in `it should die
```

## Cause

`RSpec::Expectations::ExpectationNotMetError` is not subclass of `StandardError` from RSpec 3.

see:

- [rspec-expectations v2.14.5](https://github.com/rspec/rspec-expectations/blob/v2.14.5/lib/rspec/expectations/errors.rb#L4-L6)
- [rspec-expectations v3.0.3](https://github.com/rspec/rspec-expectations/blob/v3.0.3/lib/rspec/expectations.rb#L66)