Fix depwarns for 0.7-beta

The `julia` keyword argument to `Documenter.deploydocs` (#14)

will be required in the future instead of automatically
defaulting to `julia = "nightly"
(see https://github.com/JuliaDocs/Documenter.jl/pull/712,
https://github.com/JuliaDocs/Documenter.jl/issues/526).

This commit adds `julia = "nightly"` to `Documenter.deploydocs`.