title: Octopack is not packing when using custom nuspec file
body: <p>I am attempting to configure an octopus deployment build in TFS.
  It was working okay initially, as in it would build, package up using octopack and deploy.</p>
  
  <p>However i want to configure what files get packed and realised that I need to use a custom nuspec file.</p>
  
  <p>I created a new file and used a default .nuspec file with no changes as a starting point.</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;package &gt;
    &lt;metadata&gt;
      &lt;id&gt;Package&lt;/id&gt;
      &lt;version&gt;1.0.0&lt;/version&gt;
      &lt;authors&gt;me&lt;/authors&gt;
      &lt;owners&gt;me&lt;/owners&gt;
      &lt;licenseUrl&gt;http://LICENSE_URL_HERE_OR_DELETE_THIS_LINE&lt;/licenseUrl&gt;
      &lt;projectUrl&gt;http://PROJECT_URL_HERE_OR_DELETE_THIS_LINE&lt;/projectUrl&gt;
      &lt;iconUrl&gt;http://ICON_URL_HERE_OR_DELETE_THIS_LINE&lt;/iconUrl&gt;
      &lt;requireLicenseAcceptance&gt;false&lt;/requireLicenseAcceptance&gt;
      &lt;description&gt;Package description&lt;/description&gt;
      &lt;releaseNotes&gt;Summary of changes made in this release of the package.&lt;/releaseNotes&gt;
      &lt;copyright&gt;Copyright 2015&lt;/copyright&gt;
      &lt;tags&gt;Tag1 Tag2&lt;/tags&gt;
      &lt;dependencies&gt;
        &lt;dependency id="SampleDependency" version="1.0" /&gt;
      &lt;/dependencies&gt;
    &lt;/metadata&gt;
  &lt;/package&gt;
  </code></pre>
  
  <p>With this file present and checked in when I do a build it is not creating a nuget package.</p>
  
  <p>As soon as I exclude the nuspec file it all works again.</p>
  
  <p>The nuspec file is <code>My.Project.Name.nuspec</code></p>