Do not apt-get python-gevent and python-dev

As stated in commit b5053149, these
packages are not needed for the installation of snf-image-creator.

docs/install.rst

@@ -28,8 +28,7 @@ following command:
 
 .. code-block:: console
 
-   $ apt-get install python-setuptools python-guestfs python-dialog \
-     python-gevent python-dev
+   $ apt-get install python-setuptools python-guestfs python-dialog
 
 The rest of the dependencies will be automatically resolved by setuptools.
 Keep in mind though that git needs to be installed to download the source