Update Auth0 sponsorship link (#392)

* Update Auth0 sponsorship link 

Hey

We recently launched a new page specifically geared towards developers on auth0.com.
Can we change the link in the sponsorship message?

Thanks again for your open-source work!

Sam

* Update both link and link text

README.md

@@ -13,7 +13,7 @@ Welcome all PRs for maintaining this project, or provide a link to the repositor
 
 ### Sponsor
 
-| <img width="50" src="https://user-images.githubusercontent.com/83319/31722733-de95bbde-b3ea-11e7-96bf-4f4e8f915588.png"> | <div style="text-align: left;">If you want to quickly implement SAML SSO, feel free to check out Auth0's NodeJS SDK and free plan at [auth0.com/overview](https://auth0.com/overview?utm_source=GHsponsor&utm_medium=GHsponsor&utm_campaign=samlify&utm_content=auth).</div> |
+| <img width="50" src="https://user-images.githubusercontent.com/83319/31722733-de95bbde-b3ea-11e7-96bf-4f4e8f915588.png"> | <div style="text-align: left;">If you want to quickly implement SAML SSO, feel free to check out Auth0's NodeJS SDK and free plan at [auth0.com/developers](https://auth0.com/developers?utm_source=GHsponsor&utm_medium=GHsponsor&utm_campaign=samlify&utm_content=auth).</div> |
 | :----------------------------------------------------------------------------------------------------------------------: | :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |