Only call captureStackTrace when possible

[SamVerschueren]

This PR fixes #135.

@sindresorhus You pointed to this block which does

if ('captureStackTrace' in Error) {
	Error.captureStackTrace(this, context);
} else {
	this.stack = new Error(message).stack;
}

It looks like the else clause is moot, no? So I left it out. Let me know if it isn't :).

[sindresorhus]

Seems like Travis is failing.

[sindresorhus]

It looks like the else clause is moot, no? So I left it out. Let me know if it isn't :).

It's not moot. It's useful to get a better stack trace in non-V8 environments.

[SamVerschueren]

But isn't this.stack already defined as we are extending from error? So feels like we are just overwriting it with the same value.

[sindresorhus]

Good point. You're right, it's not needed.

[SamVerschueren]

Fixed the travis errors as well.