Support GraphQL 0.12 invalid literals

[jbblanchet]

• Support returning undefined for invalid literals, like detailed in (Potentially Breaking) Allow serializing scalars as null. graphql/graphql-js#1104
• Support Kind.NULL, with required upgrade of minimum version to 0.8
• Upgraded dev dependencies
• Async unit tests return promise for better error reporting
• Added linting rule to support chai to.be.undefined

[taion]

Thanks! I'm going to make a few small tweaks in a sec

[jbblanchet]

@taion If it's quicker for you to point out here what need to be changed, I don't mind doing the changes.

[taion]

Updated, can you take a quick look?

[jbblanchet]

Looks good to me. Didn't want to refactor too much stuff so I went for the minimum 😄

Only thing that seems to be missing for a release if the version number.

[taion]

We do version bumps separately. Will get back.

[taion]

v0.2.0 released

[jbblanchet]

Thanks for the quick release!