Small Travis CI optimization

Update the manifest versions before starting the build

Updating the manifest versions after starting the build
results in 'make firefox-addon' reinstalling all of the npm
packages
Improve item list scrolling feel in Chrome

A combination of using 'transform' rather than 'top'
to position items and rendering several items above
and below what is currently visible in the viewport
reduces 'pop-in' of new items when scrolling the
item list in Chrome

Based on ideas from Polymer's <core-input> element
implementation.