minor, str.join() expects strings

With versions below minimum version, instead of raising
an informative error message the following traceback occurs:
TypeError: sequence item 0: expected string, int found

```python
>>> git checkout upstream/master
>>> Traceback (most recent call last):
  File "<string>", line 1, in ?
  File "setupinfo.py", line 300, in check_min_version
    print("Minimum required version of %s is %s, found %s" % (
TypeError: sequence item 0: expected string, int found
>>> git checkout master
>>> python -c "import setupinfo; print(setupinfo.check_min_version('1.1.17', (1, 1, 23), 'libxslt'))"
Minimum required version of libxslt is 1.1.17, found 1.1.23
False
```

Change-Id: I5ac35935cd61786181e1c6372e752a1600cc2c2d