Replace assertRaisesRegex by assertRaises

The unittest.assertRaisesRegexp function of Python2.7 has been
renamed to assertRaisesRegex in current versions of Python3.
To avoid this compatibility issue and in line with
[this issue](https://github.com/exercism/xpython/issues/30)
I replace these assertions with the assertRaises function.
Merge pull request #51 from sjakobi/fix-minesweeper

Replace assertRaisesRegex by assertRaises
    
The unittest.assertRaisesRegexp function of Python2.7 has been
renamed to assertRaisesRegex in current versions of Python3.
To avoid this compatibility issue and in line with
[this issue](https://github.com/exercism/xpython/issues/30)
I replace these assertions with the assertRaises function.