Use rspec-core instead of just rspec for rspec-rails compatibility
Use rspec-core instead of just rspec for rspec-rails compatibility

`rspec-rails` does not include the meta-gem `rspec`, thus requiring
`rspec/version` will fail.
Instead, rely on `rspec-core` which is included by both `rspec` and
`rspec-rails`.

Fixes #83

Shopify/statsd-instrument