Rely on RbConfig host_os to determine Windows

Use of RUBY_PLATFORM is not accurate or recommended. Usage of host_os
is the recommended mechanism.
Installer shouldn't be adding bundled Ruby to PATH

As stated by the installer, The Ruby VM bundled with the installer
is isolated and for the unique purposes of running 'heroku' command
line and nothing else.

Adding it to the PATH affects other Ruby installations and not only
that, it was done at machine level (HKLM) affecting other user accounts on
the same computer.

Such modification is bad and produced several issues to other projects
like RailsInstaller, RubyStack and RubyInstaller users directly.
Do not change HOME environment variable. Fixes #28

A tool should not be setting user level (HKCU) HOME environment variable
that can override the ones defined by the user or that are used
automatically by other tools.

Under some systems with roaming profiles, USERPROFILE will not be pointing
to the remote folder of the user but instead the local, volatile folder
instead.

This can cause the lost of user private keys between sessions.

Also, setting this variable affects other environments like cygwin.

Newer version of Git provides a git.exe that do not require setting such
variable to work properly.
Do not add Git's bin directory to the PATH

Adding msysGit bin directory to the PATH causes normal Windows tools like
'find' stop working.

Newer versions of msysGit provides a 'cmd' folder with 'git.exe' that
internally changes the path and lookup for posix-compatible tools without
the need to alter the PATH.
Bash: build Ruby path relative to script

Use relative-load trick using bash so invocation of 'heroku' script
captures the right version of Ruby (the bundled one) instead of any
possible version available on the user system.
Batch: Use Ruby relative to Heroku installation

Build path to bundled version of Ruby by using %HerokuPath% environment
variable already set.

Do not attempt to determine the platform as it is no needed and
RubyInstaller do not support Win9x either.

Ensure bundled Ruby goes first in the PATH avoiding any issue with any
other installation.
Install bundled Ruby relative to application

Instead of placing Ruby at Program Files level, place it relative to
application path (Heroku) to avoid possible clashes with other
installations.

Adjust Foreman and Taps gem installation to use the new location of 'gem'.