Better error when gem does not exist yet.

Bump to version 0.0.2

A more human readable error instead of
$ rake rpm:build
rake aborted!
No such file or directory - /tmp/package-gem-build20131106-25636-66nkry/pkg/foo-0.18.2.gem
/home/user/.gem/ruby/1.9.1/gems/fpm-0.4.42/lib/fpm/package/gem.rb:74:in `download'
/home/user/.gem/ruby/1.9.1/gems/fpm-0.4.42/lib/fpm/package/gem.rb:57:in `download_if_necessary'
/home/user/.gem/ruby/1.9.1/gems/fpm-0.4.42/lib/fpm/package/gem.rb:44:in `input'
/home/user/.gem/ruby/1.9.1/gems/fpm-rake-0.0.1/lib/fpm/rake/rpm_task.rb:73:in `build_rpm'
/home/user/.gem/ruby/1.9.1/gems/fpm-rake-0.0.1/lib/fpm/rake/rpm_task.rb:40:in `block (2 levels) in install'
Tasks: TOP => rpm:build
(See full trace by running task with --trace)
Append instead of replace dependencies.

Bump to version 0.0.3
rpm:dependencies task

Build an RPM of each dependency.

Not sure if this really belongs in this general package because it
assumes bundler is used.

Bump to version 0.1.0
rpm:clean task to remove pkg/*.rpm

Bump to version 0.2.0