Handle building on CentOS

On CentOS you get the following output:

    >>> import platform
    >>> platform.dist()
    ('centos', '7.5.1804', 'Core')

streamer/setup.py

@@ -4,7 +4,7 @@
 
 # In EL twisted is still has sub-packages.
 dist = platform.dist()
-if dist[0] == 'redhat' and int(float(dist[1])) <= 7:
+if dist[0] in ('redhat', 'centos') and int(dist[1].split('.', 1)[0]) <= 7:
     twisted = ['twisted-core', 'twisted-web']
 else:
     twisted = ['twisted']