Make RPM cope with an input epoch of zero.

Some RPMs from jpackage.org have their epoch set to 0, causing epoch to
raise an exception.

```
[kali:Downloads]% fpm -s rpm -t rpm --edit ant-1.8.2-5.jpp6.noarch.rpm
/Users/ben/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/gems/fpm-0.4.27/lib/fpm/package/rpm.rb:274:in `epoch': undefined method `empty?' for 0:Fixnum (NoMethodError)
	from /Users/ben/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/gems/fpm-0.4.27/lib/fpm/command.rb:342:in `block in execute'
	from /Users/ben/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/gems/fpm-0.4.27/lib/fpm/command.rb:350:in `call'
	from /Users/ben/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/gems/fpm-0.4.27/lib/fpm/command.rb:350:in `execute'
	from /Users/ben/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/gems/clamp-0.3.1/lib/clamp/command.rb:64:in `run'
	from /Users/ben/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/gems/clamp-0.3.1/lib/clamp/command.rb:126:in `run'
	from /Users/ben/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/gems/fpm-0.4.27/bin/fpm:8:in `<top (required)>'
	from /Users/ben/.rbenv/versions/1.9.3-p327/bin/fpm:23:in `load'
	from /Users/ben/.rbenv/versions/1.9.3-p327/bin/fpm:23:in `<main>'
```