21760/22005 update preflight to delete old lib dir

In order to install in the top-level site_ruby directory, we need to clean out
any installs in the old site_ruby/1.8 location. We're doing this by setting a
variable in the package:apple task. If it's set, we remove all files there as
well as files in the new location.

While I'm here:
* update erb to strip leading and trailing spaces
* validate critical variables so a script is never generated with dangerous paths
* only target top-level files/dirs in lib, since we're doing rm -RF anyway
Use Dir.glob(), don't require rubygems or rake, use Dir.chdir as a block
use instance_variable_get instead of eval
Merge pull request #512 from ccaviness/22005-install-to-top-level-site_ruby

21760/22005 update preflight to delete old lib dir