Check for existance of $INSTALL_DIR pre-install

As a fix for #56 and #57, check to see if $INSTALL_DIR is already a
directory. If it is, install the specified Ruby to
$INSTALL_DIR/$RUBY-$RUBY_VERSION. If it isn't, install directly to
$INSTALL_DIR. The default for this directory is still the same:
/opt/rubies or ~/.rubies, but this prevents problems such as specifying
`ruby-install -i ~/.rbenv/versions ruby 2.0.0` causing Ruby 2.0.0 to be
installed with ~/.rbenv/versions as the root directory rather than
~/.rbenv/versions/ruby-2.0.0-p247

Signed-off-by: David Celis <me@davidcel.is>