Update rocket in juniper_rocket to work with latest nightlies
Update rocket in juniper_rocket to work with latest nightlies

`juniper_rocket` now requires nightly >= 2018-01-12. See
https://github.com/SergioBenitez/Rocket/issues/513#issuecomment-357407524.

Fixes https://github.com/graphql-rust/juniper/issues/125.

graphql-rust/juniper,mhallin/juniper