rubocop gem install gives ERROR: Failed to build native gem extension

Question

I recently installed Ruby from http://railsinstaller.org/en. This included Ruby on Rails and the Ruby dev-kit.

Now I am trying to install rubocop, but it isn't working:

C:\Windows\System32>gem install rubocop
Temporarily enhancing PATH to include DevKit...
Building native extensions.  This could take a while...
The system cannot find the path specified.
ERROR:  Error installing rubocop:
    ERROR: Failed to build gem native extension.

current directory: C:/_Programming/Ruby/Installation/RailsInstaller/Ruby2.3.
0/lib/ruby/gems/2.3.0/gems/rainbow-2.2.2/ext
C:/_Programming/Ruby/Installation/RailsInstaller/Ruby2.3.0/bin/ruby.exe mkrf_con
f.rb

rake failed, exit code 1

Gem files will remain installed in C:/_Programming/Ruby/Installation/RailsInstal
ler/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/rainbow-2.2.2 for inspection.
Results logged to C:/_Programming/Ruby/Installation/RailsInstaller/Ruby2.3.0/lib
/ruby/gems/2.3.0/extensions/x86-mingw32/2.3.0/rainbow-2.2.2/gem_make.out

C:\Windows\System32>echo %path%
C:\ProgramData\Oracle\Java\javapath;C:\_Programming\Install\Python27\;C:\_Progra
mming\Install\Python27\Scripts;C:\Program Files (x86)\Borland\Delphi7\Bin;C:\Pro
gram Files (x86)\Borland\Delphi7\Projects\Bpl\;C:\Program Files (x86)\Embarcader
o\Studio\16.0\bin;C:\Users\Public\Documents\Embarcadero\Studio\16.0\Bpl;C:\Progr
am Files (x86)\Embarcadero\Studio\16.0\bin64;C:\Users\Public\Documents\Embarcade
ro\Studio\16.0\Bpl\Win64;C:\Ruby193\bin;C:\Windows\System32;C:\Program Files (x8
6)\Borland\Delphi5\Bin;Q:\;C:\Windows\System32\Windows System Resource Manager\b
in;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files\TortoiseSVN\bin;
C:\Program Files (x86)\Extra!\;C:\HashiCorp\Vagrant\bin;C:\_Programming\Ruby\Ins
tallation\RailsInstaller\Ruby2.3.0\bin;C:\Windows\system32\16.0\Bpl;C:\Program F
iles\Git\cmd