Update PayPal SDK Code dependency to 0.3.0

Hello!

This pull request updates PayPal SDK Core dependency to version 0.3.0

We are using `paypal-sdk-permissions` and `paypal-sdk-merchant` gems. However, we are unable to update the `paypal-sdk-merchant` gem, due to a Gem version conflict. This gem depends on Core 0.2, where as the Merchant gem depends on 0.3.0. Updating the dependency of this gem allows us to update Merchant gem and use newer PayPal API version.

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Mikko Koski
Lead Web Engineer
Sharetribe
www.sharetribe.com