Use the latest version of Firefox in CI.
Firefox versions 55.0 - 57.x had a bug that caused them to fail to take
screenshots that included img elements
(https://bugzilla.mozilla.org/show_bug.cgi?id=1409992).  Now that this
has been fixed, resume testing with the latest Firefox.
MODIFY
.travis.yml
.travis.yml
@@ -8,8 +8,8 @@ node_js:
- 4
addons:
- # firefox: latest # version 55.0 has an issue.
- firefox: 54.0.1
+ firefox: latest # version 55.0 - 57.x have an issue with screenshots.
+ # firefox: 54.0.1
chrome: stable
apt:
packages:

Use the latest version of Firefox in CI.

Firefox versions 55.0 - 57.x had a bug that caused them to fail to take
screenshots that included img elements
(https://bugzilla.mozilla.org/show_bug.cgi?id=1409992).  Now that this
has been fixed, resume testing with the latest Firefox.