Upgrade mock-fs dev dependency to support Node v12

[mcous]

Hello! After cloning this repository and installing dependencies, I found that I had two failing tests:

  71 passing (135ms)
  2 failing

  1) when modules dir does not exist should not log ENOENT error:
     Error: ENOENT, no such file or directory 'node_modules/somepackage/node_modules'
      at Binding.<anonymous> (node_modules/mock-fs/lib/binding.js:1086:13)
      at maybeCallback (node_modules/mock-fs/lib/binding.js:42:17)
      at Binding.access (node_modules/mock-fs/lib/binding.js:1083:3)
      at Object.existsSync (fs.js:240:11)
      at Object.readDir (utils.js:10:13)
      at nodeExternals (index.js:36:100)
      at Context.<anonymous> (test/library.spec.js:474:24)
      at processImmediate (internal/timers.js:456:21)

  2) when modules dir does not exist should process like node_modules is empty:
     Error: ENOENT, no such file or directory 'node_modules/somepackage/node_modules'
      at Binding.<anonymous> (node_modules/mock-fs/lib/binding.js:1086:13)
      at maybeCallback (node_modules/mock-fs/lib/binding.js:42:17)
      at Binding.access (node_modules/mock-fs/lib/binding.js:1083:3)
      at Object.existsSync (fs.js:240:11)
      at Object.readDir (utils.js:10:13)
      at nodeExternals (index.js:36:100)
      at Context.<anonymous> (test/library.spec.js:484:24)
      at processImmediate (internal/timers.js:456:21)

I'm running the latest Node LTS version (v12), so I switched over to the last LTS version to successfully run in CI for this repository (v8) and found that tests passed locally. I tracked down the issue to a problem with the mock-fs dependency: tschaub/mock-fs#256

This PR upgrades that dependency so that tests will pass on the latest version of Node. This PR also introduces a fair amount of lockfile churn, but I suspect that's because the lockfile hasn't been touched in a while. I'm running the latest version of the npm cli (v6.14.5).