Set BUNDLE_PATH to match GEM_HOME

Hi ArchivesSpacers,

Doing some development locally, we had a case where a BUNDLE_PATH variable (set by RVM) caused the "bootstrap" to install the gems into the wrong directory.  Hilarity ensued :)

This commit sets the variable to match GEM_HOME, which seems to fix things.  Does this seem reasonable to you?

Thanks,
Mark