Fix version key typo on install.js

[kevinzwhuang]

Summary

yarn install is broken for the latest nightly, v0.18.0 - introduced by a typo from #1429
There is a typo in install.js in the latest nightly where the version key for yarn is spelled as verison.

const {verison: YARN_VERSION, installationMethod: YARN_INSTALL_METHOD} = require('../../../package.json');

This results in YARN_VERSION being undefined and causing errors down the script. Such as:

Trace:
  TypeError: Cannot read property 'indexOf' of undefined
      at Install.checkUpdate (/usr/share/yarn/lib/cli/commands/install.js:906:21)
      at /usr/share/yarn/lib/cli/commands/install.js:466:14
      at next (native)
      at step (/usr/share/yarn/node_modules/babel-runtime/helpers/asyncToGenerator.js:17:30)
      at /usr/share/yarn/node_modules/babel-runtime/helpers/asyncToGenerator.js:35:14
      at Promise.F (/usr/share/yarn/node_modules/core-js/library/modules/_export.js:35:28)
      at /usr/share/yarn/node_modules/babel-runtime/helpers/asyncToGenerator.js:14:12
      at Install.init (/usr/share/yarn/lib/cli/commands/install.js:580:7)
      at /usr/share/yarn/lib/cli/commands/install.js:55:21
      at next (native)

Test plan

Simply fixing the typo to version corrects the behavior for yarn install, as such:

const {version: YARN_VERSION, installationMethod: YARN_INSTALL_METHOD} = require('../../../package.json');

---

cc: @kittens @bestander

[kevinzwhuang]

Thanks @bestander!