fix DEPRECATION warning from RSpec.

I began to use parslet, it looks really cool.
Unfortunately, it doesn't pass spec well. I got no failure but some warnings with running `rspec` command:

```log
Run options: exclude {:ruby=>#<Proc:./spec/spec_helper.rb:13>}
...........................................DEPRECATION: `expect { }.not_to raise_error(SpecificErrorClass)` is deprecated. Use `expect { }.not_to raise_error` (with no args) instead. Called from /home/tadashi/git/parslet/spec/acceptance/regression_spec.rb:87:in `block (4 levels) in <top (required)>'.
.......................................................................................................................................................................DEPRECATION: `expect { }.not_to raise_error(SpecificErrorClass)` is deprecated. Use `expect { }.not_to raise_error` (with no args) instead. Called from /home/tadashi/git/parslet/spec/parslet/convenience_spec.rb:21:in `block (4 levels) in <top (required)>'.
.DEPRECATION: `expect { }.not_to raise_error(SpecificErrorClass)` is deprecated. Use `expect { }.not_to raise_error` (with no args) instead. Called from /home/tadashi/git/parslet/spec/parslet/convenience_spec.rb:24:in `block (4 levels) in <top (required)>'.
...................................................................................................................................................................................................

Finished in 5.08 seconds
406 examples, 0 failures
```

I agree with the warnings because it seems that raising nothing is correct spec, so I removed class specification.

My rspec version is:

    $ rspec --version
    2.14.6