Added cowlib and ranch as dependencies to fix the `mix deps.compile` step

When I tried to build all dependencies I get errors 
```
[1] $ mix deps.compile                                                        ✘
* Compiling cowboy
==> cowboy (compile)
Dependency not available: cowlib-.* ({git,
                                      "git://github.com/extend/cowlib.git",
                                      "0.3.0"})
Dependency not available: ranch-.* ({git,"git://github.com/extend/ranch.git",
                                     "0.8.5"})
ERROR: compile failed while processing ws-elixir/deps/cowboy: rebar_abort
** (Mix) Could not compile dependency cowboy, rebar command failed. If you want to recompile this dependency, please run: mix deps.compile cowboy
```

Adding "cowlib" and "ranch" to the dependencies definition in *mix.exs* fixed it.