fix: replace deprecated client.request method call with colon syntax

[ilovemikael]

I don't have much else to say; sorry if this is already being worked on elsewhere (as the deprecation warnings have been present for months now, not just an issue with this project either), but I couldn't find it being discussed anywhere.

[w0rp]

Do we have a way of checking which versions of Neovim this will work with? I'd like to ensure we can be both backwards and forwards compatible.

[ilovemikael]

W.r.t. forwards compatibility, I use nvim built against the latest git master branch and have been using my fork of this repo to remove the deprecation warnings; ale has still worked fine without a hitch for the last two weeks or so of doing that.

Sorry for not providing documentation referencing the deprecation, it can be found here (scroll down to below deprecated in 0.11, it only lets you link to changes for 0.12 =( ). Since the methods were deprecated in 0.11, I'd say any version ≥0.11 would be supported.

There's a PR covering basically the same topic which you can look at for reference; the creator of that said colon syntax is supported since nvim 0.5 but later changed it to use the regular syntax for ≤0.10 to avoid some errors that I'm really not knowledgeable of, you should probably just review it yourself.