Fix eclipse update site

[KengoTODA]

This changes fix problem in Eclipse Update site reported at spotbugs/spotbugs.github.io#14

site.xml generated in this branch

<?xml version="1.0" encoding="UTF-8"?>
<site>

   <description url="https://spotbugs.github.io/eclipse">
      This is the Eclipse update site for SpotBugs features.
   </description>

   <feature url="features/com.github.spotbugs.plugin.eclipse_3.1.0.20170517-1494985961678.jar"
            id="com.github.spotbugs.plugin.eclipse"
            version="3.1.0.20170517-1494985961678">
      <category name="SpotBugs"/>
   </feature>

   <category-def name="SpotBugs" label="SpotBugs">
      <description>
         This is the Eclipse category for SpotBugs features.
      </description>
   </category-def>

</site>

[KengoTODA]

To test this groovy script, you can have a try at https://groovyconsole.appspot.com/edit/5082257685479424

[iloveeclipse]

I have no time for playing with gradle, if the fix is working, I'm fine with that.