reset color before displaying error message

Mocha (and probably other test libraries) makes the error text red when it catches something. This makes assertive's error message red up to the point where (for example) it's turned green for the expected value. This ruins the effect.

This change clears color information before displaying the error message, which can then control its own color codes.

```
[RED]Error: Assertion failed: The response is OK
Expected: [GREEN]200
[CLEAR]Actually: [RED]500
```

Above, `Expected:` is red instead of clear. With this change, it looks like:

```
[RED]Error: Assertion failed: The response is OK
[CLEAR]Expected: [GREEN]200
[CLEAR]Actually: [RED]500
```