Do you use Bio.jl or GenomicFeatures.jl?
Bio.jl is still under a large reconstruction and does not support Julia 0.6, but GenomicFeatures.jl already support it.
julia> using GenomicFeatures
julia> reader = open(BigWig.Reader, "test/BioFmtSpecimens/BBI/bigWig1.bw");
So, GenomicFeatures.jl is a recommended package now. In the near future, Bio.jl will be build on top of modular packages including GenomicFeatures.jl.