Use gdb.VERSION instead of parsing gdb.execute result (#467)

Yay, this is finally there:

```
>>> gdb.VERSION
'8.1.0.20180409-git'
```

pwndbg/commands/version.py

@@ -20,7 +20,10 @@ from pwndbg.color import message
 
 
 def _gdb_version():
-    return gdb.execute('show version', to_string=True).split('\n')[0]
+    try:
+        return gdb.VERSION  # GDB >= 8.1 (or earlier?)
+    except AttributeError:
+        return gdb.execute('show version', to_string=True).split('\n')[0]
 
 
 def _py_version():