fix(targets): Linux/ia32 is no longer officially supported for Electron >= 4

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

See:

• https://electronjs.org/blog/linux-32bit-support
• Electron 4.0.x: does not run on Ubuntu 14/16 32bits anymore electron#16407

[codecov]

Codecov Report

Merging #957 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #957   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         671    671           
=====================================
  Hits          671    671

Impacted Files	Coverage Δ
targets.js	100% <ø> (ø)	⬆️

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[Nikeron]

Linux only? Win32 will be alive? Are you seriously? 👏

[malept]

All platform/arch decisions are up to the Electron group in charge of releases, Electron Packager only follows their direction.

In this case, I'm probably going to revert this PR and release a new version. My understanding is that a warning will be emitted from electron-download when 32-bit Linux builds >= 4.0.0 are downloaded, so it's at the developer's risk to use them. (They may or may not work.)