Nitroshare python extension for Caja incorrectly uses 3.0 version of gi

Steps to reproduce:
1. Install Ubuntu MATE 19.10
2. Install all caja python extensions as follows:

     sudo apt install caja-owncloud caja-admin folder-color-caja caja-rename caja-mediainfo deja-dup-caja caja-nextcloud nitroshare-caja

3. Restart Caja with `caja -q` while controlling output of `tail -f ~/.xsession-errors`

Expected results:
* there are no warnings for Caja and Python

Actual results:
* there are the following errors in the log:

```
Traceback (most recent call last):
  File "/usr/share/caja-python/extensions/nitroshare.py", line 35, in <module>
    gi.require_version('Caja', '3.0')
  File "/usr/lib/python2.7/dist-packages/gi/__init__.py", line 121, in require_version
    (namespace, loaded_version))
ValueError: Namespace Caja is already loaded with version 2.0
```

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: nitroshare-caja 0.3.3-1.1
ProcVersionSignature: Ubuntu 5.3.0-18.19-generic 5.3.1
Uname: Linux 5.3.0-18-generic x86_64
ApportVersion: 2.20.11-0ubuntu8
Architecture: amd64
CurrentDesktop: MATE
Date: Sun Oct 20 22:28:24 2019
InstallationDate: Installed on 2019-10-17 (2 days ago)
InstallationMedia: Ubuntu-MATE 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
PackageArchitecture: all
SourcePackage: nitroshare
UpgradeStatus: Upgraded to eoan on 2019-10-18 (2 days ago)