Code where we call a method of something which isn't a variable name
e.g.

```
yesterday = (date.today() - timedelta(days=1)).strftime("%Y-%m-%d")
```

was causing pyt to crash.

Previously the else branch would only handle ast.Attribute, and crash on
everything else.

```
Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/pyt/pyt/__main__.py", line 141, in <module>
    main()
  File "/pyt/pyt/__main__.py", line 97, in main
    path
  File "/pyt/pyt/cfg/make_cfg.py", line 40, in make_cfg
    module_definitions
  File "/pyt/pyt/cfg/expr_visitor.py", line 60, in __init__
    self.init_cfg(node)
  File "/pyt/pyt/cfg/expr_visitor.py", line 67, in init_cfg
    module_statements = self.visit(node)
  File "/usr/lib/python3.6/ast.py", line 253, in visit
    return visitor(node)
  File "/pyt/pyt/cfg/stmt_visitor.py", line 58, in visit_Module
    return self.stmt_star_handler(node.body)
  File "/pyt/pyt/cfg/stmt_visitor.py", line 79, in stmt_star_handler
    node = self.visit(stmt)
  File "/usr/lib/python3.6/ast.py", line 253, in visit
    return visitor(node)
  File "/pyt/pyt/cfg/stmt_visitor.py", line 413, in visit_Assign
    return self.assignment_call_node(label.result, node)
  File "/pyt/pyt/cfg/stmt_visitor.py", line 437, in assignment_call_node
    call = self.visit(ast_node.value)
  File "/usr/lib/python3.6/ast.py", line 253, in visit
    return visitor(node)
  File "/pyt/pyt/cfg/expr_visitor.py", line 540, in visit_Call
    _id = get_call_names_as_string(node.func)
  File "/pyt/pyt/core/ast_helper.py", line 78, in get_call_names_as_string
    return _list_to_dotted_string(get_call_names(node))
  File "/pyt/pyt/core/ast_helper.py", line 68, in get_call_names
    return reversed(_get_call_names_helper(node, result))
  File "/pyt/pyt/core/ast_helper.py", line 62, in _get_call_names_helper
    return _get_call_names_helper(node.value, result)
  File "/pyt/pyt/core/ast_helper.py", line 61, in _get_call_names_helper
    result.append(node.attr)
AttributeError: 'BinOp' object has no attribute 'attr'
```