Bug 960039 - [system] Remove `charging` attribute

Remove unused, non-standard `charging` attribute from the markup for the
battery icon.
Merge pull request #15386 from zacc/bug_946205

Bug 946205 - Re-enable Travis tests after 891882 merged
Merge pull request #15355 from mmalecki/remove-charging-attribute

Bug 960039 - [system] Remove `charging` attribute, r=timdream
Bug 955626 - Change start_b2g wait in GaiaTest to wait for AppWindowManager to acknowledge the active frame instead of js event