Nothing happens on MarkdownPreview

Thanks for this plugin!

When I run `MarkdownPreview` on a `.md` in neovim v0.3.4 nothing happens for me after I install this plugin and I'm not sure how to debug it.

I have installed the package with `dein` and use the default config from the Readme.

Running `checkhealth` produces,
```
health#mkdp#check
========================================================================
- INFO: Platform: linux
- INFO: Nvim Version: NVIM v0.3.4
- INFO: Node version: v10.15.2

- INFO: Script: /home/rth/.cache/dein/repos/github.com/iamcco/markdown-preview.nvim/app/server.js
- INFO: Script exists: 1
- OK: Using node
```
and I also have tried adding the following to `.config/nvim/init.vim`
```
let $NVIM_MKDP_LOG_FILE = expand('~/mkdp-log.log')
let $NVIM_MKDP_LOG_LEVEL = 'debug'
```
but this file is not created. Same thing happens (i.e. nothing) when I set,
```
let g:mkdp_auto_start = 1
```
and try to edit/save markdown file.

Is there any other things I should try to debug it (or log files that I can look at)? Thank you!

**Edit:** another possibly relevant information is that
- I am using neovim on a remote Ubuntu 19.04 server.
- I can see that `MarkdownPreview` command is only available for markdown files, running it on any other file produces "E492: Not an editor command: MarkdownPreview", so at least the file type detection is working.