Fix to run specs on Travis CI

`rspec` handle files named as `_spec.rb` not `*_specs.rb`.

Before
```sh
$ bundle exec rspec spec
No examples found.


Finished in 0.00007 seconds
0 examples, 0 failures
```

After:
```sh
$ bundle exec rspec spec
.................................................................................

Finished in 0.01655 seconds
81 examples, 0 failures