Use thread_safe gem version greater or equal to 0.3.4

Hi, 

To fix a problem with old version of `thread_safe` gem which might cause an errors like:

```
NoMethodError: undefined method `fetch_or_store' for #<ThreadSafe::Cache:0x007fc890c902d8>
```

I'm suggesting to use `thread_safe` gem with version greater or equal to 0.3.4, when the method `#fetch_or_store` was added https://github.com/ruby-concurrency/thread_safe/commit/aed23e870be68fbac2d61731f715a1add044b226