title: Compilation error after upgrading from ACRA 4.5 to ACRA 4.6
body: <p>I upgraded the ACRA library used by my application from
  4.5.0 to 4.6.0 but I am getting a compilation error for formKey 
  which used to work with 4.5.</p>
  
  <p>Here is the declaration:</p>
  
  <pre><code>@ReportsCrashes(
    formKey = "",
    formUri = "",
    socketTimeout = 10000,
    httpMethod = org.acra.sender.HttpSender.Method.POST,
    mode = ReportingInteractionMode.SILENT,
    resDialogText = R.string.crash_dialog_text,
    resDialogIcon = android.R.drawable.ic_dialog_info,
    resDialogTitle = R.string.crash_dialog_title,
    resDialogCommentPrompt = R.string.crash_dialog_comment_prompt,
  )
  </code></pre>
  
  <p>Here is the error:</p>
  
  <blockquote>
    <p>Error:(19, 17) error: cannot find symbol method formKey()</p>
  </blockquote>
  
  <p>I tried removing the formKey, as it is not used anymore, 
  from the declaration but it's producing a new error after I did that.</p>