Fixing situation when assert has no explanation

I believe that this small change should fix the issue described in #8.

CPython handles this in a way that shows just 

    AssertionError

whereas after applying this pull request skulpt will show

    AssertionError:

What do you think about that? Is it worth changing?

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Obviously, something like

    assert 1 == 0, 'this should fail'

will result into

    AssertionError: this should fail on line 1