Viewports width broken using Drone.io CI

Hi,

Just tried to integrate backstopJs with [Drone.io](https://github.com/drone/drone) and when viewport width goes higher than some numbers it will break the tests. The Same test will pass locally.
Here is my config:

```
module.exports = {
id: 'prod_test',
viewports: [
{
name: 'browser',
width: 1366,
height: 768,
},
],
scenarios: [
{
label: 'Default',
url: 'http://localhost:3000/',
hideSelectors: [],
removeSelectors: [],
selectorExpansion: true,
selectors: [
'#root',
],
readyEvent: null,
delay: 500,
misMatchThreshold: 0.1,
requireSameDimensions: true,
},
],
paths: {
bitmaps_reference: 'src/visualTests/backstop_data/bitmaps_reference',
bitmaps_test: 'src/visualTests/backstop_data/bitmaps_test',
html_report: 'src/visualTests/backstop_data/html_report',
ci_report: 'src/visualTests/backstop_data/ci_report',
},
ci: {
format: 'junit',
},
casperFlags: [],
engine: 'phantomjs',
report: ['CI'],
debug: false,
};
```

Any idea why that would happen?
Thanks