Enable to run tests with `rspec`

Currently we gets `NameError` when we run tests by `rspec` command. Moving requiring `spec_helper` from Rakefile to .rspec makes it possible to run tests by `rspec` in addition to `rake spec`. 

I prefer to run tests by `rspec` command because I can easily specify one file like

```
$ bundle exec rspec spec/path/to/hoge_spec.rb
```

although in the case of rake spec, I have to do like

```
$ bundle exec rake spec SPEC=spec/path/to/hoge_spec.rb
````

`SPEC=` is bothersome.