Avoid bleeding BUNDLE_ env vars into gem push

Bundler intermittently fails to resolve ruby_gntp when trying to push the bosh_vcloud_cpi gem, it should not be involved in a gem push

Signed-off-by: Caleb Miles <cmiles@pivotallabs.com>
Update README to reflect that we are publishing to RubyGems

Signed-off-by: Caleb Miles <cmiles@pivotallabs.com>