Install requirements if using pip

Install requirements if using pip

Current setup was using distutils which does not allow to define the
requirements of a package. Check if the installation of
`suricata-update` is being done with `pip` and if it is, install the
requirements while installing the package.
This way distutils will not throw a warning of the `install_requires`
option being unrecognized, however, it would still not install the
requirements.
Now, with the installation of `suricata-update` package, all the
requirements are installed as well if it is installed with `pip`.

Closes redmine ticket #2667