Fix register_comm invalidations

[halleysfifthinc]

Minor bug: register_comm gets invalidated by design, when other packages (WebIO, JSServe, and Bonito that I'm aware of) define a new register_comm method.

Invalidation reproducer (from Jupyter):

using IJulia, SnoopCompileCore

invs = @snoop_invalidations using WebIO;

using SnoopCompile, AbstractTrees
trees = invalidation_trees(invs)

which gives

 inserting register_comm(comm::IJulia.CommManager.Comm{:webio_comm}, x) @ IJuliaExt ~/.julia/dev/WebIO/ext/IJuliaExt.jl:19 invalidated:
   backedges: 1: superseding register_comm(comm::IJulia.CommManager.Comm, data) @ IJulia.CommManager ~/Dropbox/documents/technology/development/julia-dev/IJulia/src/comm_manager.jl:92 with MethodInstance for register_comm(::IJulia.CommManager.Comm, ::IJulia.Msg) (1 children)

This PR changes the register_comm call to use invokelatest. This shouldn't be a performance sensitive issue, since register_comm should normally only be called once per comm connection.