fix(power-assert): use CommonJS style default export statement

[teppeis]

If ES6 style import statement is used in .ts, an assertion expression is compiled to the format that is not supported by power-assert like power_assert_1.default.ok().

For examlple,

// original .ts
import assert from 'power-assert';
import sut from '../';

describe('foo', () => {
    it('test #1', () => {
        assert.ok(sut() === 2);
    });
});

is compiled to

// generated .js
var power_assert_1 = require('power-assert');
var _1 = require('../');
describe('foo', function () {
    it('test #1', function () {
        power_assert_1.default.ok(_1.default() === 2);
    });
});

To resolve this problem, this PR changes to use CommonJS Style default export simply.

[dt-bot]

power-assert/power-assert.d.ts

to author (@vvakame). Could you review this PR?
👍 or 👎?

Checklist

• pass the Travis CI test?

[teppeis]

cc/ @twada

[twada]

@teppeis Thank you so much!!

[vvakame]

👍

[teppeis]

@vvakame thanks!