Create lockfile when missing even when integrity hash matches

[wyze]

Summary

This solves #1876. Before, if integrity hash from disk matched the calculated hash from the patterns from the manifest, then it would bail on the rest of the installation. Added a check to not bail out if the yarn.lock file does not exist. This was a regression in 0.17.x as it would create it if missing in 0.16.x.

Test plan

Setup

$ mkdir yarn-test && cd $_

$ yarn add left-pad
yarn add v0.18.0-0
info No lockfile found.
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
[4/4] 📃  Building fresh packages...
success Saved lockfile.
success Saved 1 new dependency.
└─ left-pad@1.1.3
✨  Done in 0.69s.

$ rm -rf yarn.lock

$ ls
node_modules package.json

Before

$ yarn
yarn install v0.17.2
info No lockfile found.
warning No license field
[1/4] 🔍  Resolving packages...
success Already up-to-date.
✨  Done in 0.59s.

$ ls
node_modules package.json

After

$ yarn
yarn install v0.18.0-0
info No lockfile found.
warning No license field
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
[4/4] 📃  Building fresh packages...
success Saved lockfile.
✨  Done in 0.46s.

$ ls
node_modules package.json yarn.lock

[wyze]

Travis is having issues with the NodeSource Debian repository.