Fix path of gem binstubs

Since rubygems v2.4.2, Windows binstubs for gem executables
are generated with a hardcoded path to Ruby. This leads to
the binstubs shipping with RailsInstaller-Windows being
unusable, because they reference the Ruby path on the maintainer
machine, rather than the machine that executed the installer
(see railsinstaller#81).

This is due to a fix in rubygems, which explicitely hardcodes
the Ruby path in the binstubs, to work around a Bundler issue
(see rubygems/rubygems#942).

The author of the patch notes that it makes Ruby installations
non-portable across machines - which is exacly the issue that
we encounter there. The intended workaround for people needing
portable installations is to re-generate the binstubs locally,
by executing `gem pristine --all --executables-only`.

This is how this PR implements the fix: it simply re-generates
the binstubs with the correct path at the end of the
installation.

In the future, I may attempt to fix the issue upstream in
rubygems, by allowing `--env-shebang` to generate a portable
Ruby path again, as it was suggested in the original pull
request.

resources/railsinstaller/railsinstaller.iss

@@ -130,6 +130,7 @@ Name: {group}\{cm:UninstallProgram,{#InstallerName}}; Filename: {uninstallexe}
 
 [Run]
 Filename: "{app}\{#RubyPath}\bin\ruby.exe"; Parameters: "dk.rb install --force"; WorkingDir: "{app}\DevKit"; Flags: runhidden
+Filename: "{app}\{#RubyPath}\bin\gem.bat"; Parameters: "pristine --all --only-executables"; WorkingDir: "{app}\{#RubyPath}\bin"; Flags: runhidden
 Filename: {sys}\cmd.exe; Parameters: /E:ON /K {app}\{#RubyPath}\setup_environment.bat {app}; WorkingDir: {sd}\Sites; Description: "Configure git and ssh when installation has completed."; Check: InstallGit; Flags: postinstall nowait skipifsilent
 
 ; TODO: Instead of running the full vcredist, simply extract and bundle the dll