master version of the book is not up-to-date with master

[samuela]

Describe the bug
I just noticed that the master version of the book (https://graphql-rust.github.io/juniper/master/index.html and linked to from https://github.com/graphql-rust/juniper/blob/master/README.md) is not actually up-to-date with the contents of docs/book.

To Reproduce
In particular the code blocks in https://graphql-rust.github.io/juniper/master/schema/schemas_and_mutations.html and https://github.com/graphql-rust/juniper/blob/master/docs/book/content/schema/schemas_and_mutations.md are not the same.

Expected behavior
The master book and master branch to be in alignment (via CI, etc).

Additional context
The book is very helpful! Thank you!

[LegNeato]

I believe this should be fixed when #543 lands.