Update opn to latest release

[strarsis]

Recent opn works correctly in WSL/Bash on Windows, closes #1581.

[caseywatts]

Not sure why appveyor failed - maybe worth rebasing on master and having it run again?

[caseywatts]

oh actually appveyor failed on the last PR that got merged, too

@shakyShane are we ignoring that appveyor check lately?

[strarsis]

@caseywatts, @shakyShane: The appveyor test apparently has an issue:

Using: crossbow.yaml
+ build-all
+ Results from 'build-all'
└─┬ build-all <series>
  ├─┬ lint
  │ └── x @npm prettier lib/**/* examples/* test/specs/**/* test/specs/*.js client/lib/* client/lib/messages/* client/lib/dom-effects/* client/lib/effects/* client/lib/listeners/* --write --tab-width 4 (0.03s)
  │     '"prettier lib/**/* examples/* test/specs/**/* test/specs/*.js client/lib/* client/lib/messages/* client/lib/dom-effects/* client/lib/effects/* client/lib/listeners/* --write --tab-width 4' is not recognized as an internal or external command,
  │     operable program or batch file.
  │     
  ├─┬ build-server
  │ └── x @npm tsc (didn't start)
  └─┬ build-client <series>
    ├── x @npm cd client && yarn (didn't start)
    ├── x @npm cd client && ./node_modules/.bin/webpack (didn't start)
    └── x @npm cd client && ./node_modules/.bin/webpack -p --output-filename index.min.js (didn't start)

prettier command isn't found in the appveyor environment.

[caseywatts]

prettier is in devDependencies - I wonder if appveyor does NODE_ENV=production by default, so those don't get installed?

• one way to fix this could be getting devDependencies to be installed on appveyor (NODE_ENV=test in appveyor could do that - probably anything other than NODE_ENV=production ?)
• a quicker fix might be moving prettier to dependencies (even though it doesn't belong there technically)

This is how Heroku deals with devDependencies in a testing environment - it makes sure to include those during CI:
heroku/heroku-buildpack-nodejs#519