Let Travis also execute "skipped" tests

Several test suites use the 'unittest.skip' decorator to allow
students to progressively implement features in their solutions.
So far, Travis, too, skipped the decorated test cases, with the result
that these went entirely untested.

This commit fixes this in two steps:
1. The Travis test script will now set the environment variable
   'NO_SKIP'.
2. The unittest.skip decorator is replaced by the unittest.skipUnless
   decorator that will skip tests as usual, unless 'NO_SKIP' is set.
Merge pull request #49 from sjakobi/fix-test-skips

Let Travis also execute "skipped" tests
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.
Let Travis also execute "skipped" tests

This commit adapts the remaining test suites that make use
of test skipping.
Merge pull request #50 from sjakobi/fix-test-skips

Let Travis also execute "skipped" tests