The Travis log shows here
ERROR: LoadError: MethodError: no method matching joinpath()
Closest candidates are:
joinpath(::AbstractString) at path.jl:244
joinpath(::AbstractString, ::AbstractString...) at path.jl:290
so that’s presumably the source of your build failure.
The Documenter.jl docs are quite good I think, so you should review those to see how to docs get build and deployed. If you can successfully build your documentation locally, there are only a small additional number of steps to get it to deploy during a CI build.