OnInstall package using HttpRequest or Callout creates System.CalloutExceptio

Question

I've tried alot of solutions for this issue but when I install the package then I received an email with exception info as below:

System.CalloutException: You have uncommitted work pending. Please commit or rollback before calling out.

Please find below my code:

global class PostInstallClass implements InstallHandler {
  global void onInstall(InstallContext context) {
      User u = GetUserById(context.installerId());
      try
      {
          if(!Test.isRunningTest())          UpdatePackageInstallationInfo(context.installerId(), u.Email);
      }
      catch(Exception e)
      {
          SendMessage(new String[]{'[email protected]'},'Package failed to install/upgrade','Failed to install/upgraed package. ' + u.Email + ' tried to install/upgraed the package but failed because. Error Details: ' + e);
      }     
  }

    public User GetUserById(ID id)
    {
        User u = [Select Id, Email from User where Id =:id];
        return u;
    }

    @future(callout=true)
    public static void UpdatePackageInstallationInfo(String organizationId, String userEmail)
    {        
        String url = 'http://demoapi.sunapplabs.com/api/salesforce/updateinstallpackage?organizationid='+organizationId;
        Http h = new Http();
        HttpRequest req = new HttpRequest();
        req.setEndpoint(url);
        req.setMethod('GET');
        HttpResponse responseResult = h.send(req);
        if(responseResult.getBody() == 'true')
        {            
            SendMessage(new String[]{userEmail, '[email protected]'},'Package install successful','Thanks for installing the package. Response result : '+responseResult);
        }
        else
        {
            SendMessage(new String[]{'[email protected]'},'Package failed to install/upgrade','Failed to install/upgraed package. ' + userEmail + ' tried to install/upgraed the package but failed because of failure from MyAPI response. Response result : '+responseResult);
        }
    }
    public static void SendMessage(String[] toAddresses, String subject, String message)
    {
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        mail.setToAddresses(toAddresses);
        mail.setReplyTo('[email protected]');
        mail.setSenderDisplayName('My Package Support');
        mail.setSubject(subject);
        mail.setPlainTextBody(message);
        Messaging.sendEmail(new Messaging.Email[] { mail }); 
    }
  }