System check doesn't work with yarn nightlies

[malept]

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Please describe your issue:

Yarn nightlies don't follow semver, so the package manager version check fails. It will fail on any Forge subcommand because the system check always runs.

Example output (on Windows):

An unhandled error has occurred inside Forge:
Invalid Version: 1.1.0-20170925.1016

TypeError: Invalid Version: 1.1.0-20170925.1016

   at new SemVer (C:\Users\IEUser\projects\electronapp\node_modules\semver\semver.js:293:11)
   at Range.test (C:\Users\IEUser\projects\electronapp\node_modules\semver\semver.js:1100:15)
   at Function.satisfies (C:\Users\IEUser\projects\electronapp\node_modules\semver\semver.js:1149:16)
   at warnIfPackageManagerIsntAKnownGoodVersion (C:\Users\IEUser\projects\electronapp\node_modules\electron-forge\dist\util\check-system.js:111:25)
   at C:\Users\IEUser\projects\electronapp\node_modules\electron-forge\dist\util\check-system.js:72:17
   at process._tickCallback (internal/process/next_tick.js:103:7)
error Command failed with exit code 1.

[malept]

Talked to @MarshallOfSound and we should warn on any nightly version.