Fix grange.from_text()

Fix grange.from_text()

Would be possible to use regexp instead?

Fixing this error from tests:

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ERROR: testFailFromText2 (test_grange.GRangeTestCase)
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Traceback (most recent call last):
  File "~/dnspython/tests/test_grange.py", line 81, in testFailFromText2
    self.assertRaises(dns.exception.SyntaxError, bad)
  File "/usr/lib64/python2.7/unittest/case.py", line 511, in assertRaises
    callableObj(*args, **kwargs)
  File "~/dnspython/tests/test_grange.py", line 80, in bad
    dns.grange.from_text('%s-%d/%d' % (start, stop, step))
  File "~/dnspython/dns/grange.py", line 39, in from_text
    start = int(cur)
ValueError: invalid literal for int() with base 10: ''