Tweak POST to use pure POST api

Instead of multipart nonsense, use the undocumented form-data POST API

Note that I haven't tested this, since I'm not sure how to run this Coverage.jl thing, but in a REPL session, the following returned me something:

```julia
julia> data
{"source_files"=>[{"coverage"=>{nothing,1,nothing},"name"=>"example.rb","source"=>"def four\
n  4\nend"},{"coverage"=>{nothing,1,0,nothing},"name"=>"lib/two.rb","source"=>"def seven\n  
eight\n  nine\nend"}],"repo_token"=>"NnS70s2bP8Wwvxquax6rdRz4zDnEwW5OK"}

julia> r = Requests.post(URI("https://coveralls.io/api/v1/jobs"), data = {"json" => json(dat
a)}, headers={"Content-Type" => "text/form-data"})
<snip a bunch of deprecation warnings>
Response(200 OK, 20 Headers, 66 Bytes in Body)

julia> r.data
"{\"message\":\"Job #3.1\",\"url\":\"https://coveralls.io/jobs/1754807\"}\r\n"
```