'zip' gem conflicts with 'rubyzip' on Redmine XLS Export Plugin.

### What steps will reproduce the problem?
1. Install DMSF Plugin and XLS Export Plugin.
http://www.redmine.org/plugins/redmine_xls_export
2. Do following because XLS Export Plugin needs Zip::ZipOutputStream::write_buffer.
<pre>
$ irb
>> require 'zip/zip'
=> true
>> p defined?(Zip::ZipOutputStream::write_buffer))
=> nil
</pre>

### What is the expected output? What do you see instead?
Now following issue is occurred. I wanna fix it.
https://github.com/two-pack/redmine_xls_export/issues/19

### What version of the product are you using? On what operating system?
#### Environment:
  Redmine version                          2.2.4.stable
  Ruby version                             1.8.7 (universal-darwin11.0)
  Rails version                            3.2.13
  Environment                              development
  Database adapter                         MySQL
#### Redmine plugins:
  redmine_dmsf                             1.4.5 stable
  redmine_plugin_views_revisions           0.0.1
  redmine_xls_export                       0.2.1

### Please provide any additional information below.
When loading init.rb in XLS Export, $: has paths of 'zip' and 'rubyzip'.
Index of 'zip' in $: is earlier then 'rubyzip'. And DSMF and XLS Export are same condition.
So 'zip' is loaded first, and same name library ISN'T loaded with require.

By the way, I checked DMSF with 'rubyzip', of course not all the things, maybe no problem.
I pull-request and want to change 'zip' to 'rubyzip' on your plugin if you can.