copr: actually invoke rstrip

Previously it could throw:
```
AttributeError: 'builtin_function_or_method' object has no attribute 'split'
```
Closes: #421
Approved by: xsuchy

plugins/copr.py

@@ -50,7 +50,7 @@ def linux_distribution():
             with open('/etc/os-release') as os_release_file:
                 os_release_data = {}
                 for line in os_release_file:
-                    os_release_key, os_release_value = line.rstrip.split('=')
+                    os_release_key, os_release_value = line.rstrip().split('=')
                     os_release_data[os_release_key] = os_release_value.strip('"')
                 return (os_release_data['NAME'], os_release_data['VERSION_ID'], None)