Error while building (pangocairo)

## Issue or Feature
Issue

Build fails for canvas on travis/heroku (looks like a error while trying to find pangocairo)
## Steps to Reproduce

```js
var Canvas = require('canvas');
var canvas = Canvas.createCanvas(200, 200);
var ctx = canvas.getContext('2d');
// etc.
```
Build it on travis or another CLI

## Your Environment
* Version of node-canvas (e.g. 1.4.0):
* Environment (e.g. node 4.2.0 on Mac OS X 10.8):
canvas@2.0.0-alpha.17
Ubuntu 18.04