Update installation steps on README.md file

[ghost]

The README.md file currently references:

• version 0.4.0, but latest version is 0.8.2
• that the package name is gremlin_client, but it is now gremlin-client

[wolf4ood]

Thanks @augmented-fog

I've just pushed a fix

[ghost]

Oh wow, that fix was quick. Thank you so much.