Minor Updates to Scrollspy.js

[mikegreiling]

Minor refactoring...

• Make this.offsets and this.targets vanilla javascript arrays again
• Removes check for href attribute on the scrollable element – No one is going to use an <a> tag to define a scrollable block
• Removes this.$element since it was merely a duplicate of this.$scrollElement
• Combine logic of multiple segments within the refresh method which depend on whether or not $scrollElement is equal to window for better speed and readability.

[cvrebert]

Please fix the violations of our coding style rules; see the Travis build failure for details.

[mikegreiling]

Should be fixed now... how do I rerun the Travis build?

Edit: Nevermind, seems to be going now...

[fat]

code, lgtm

[cvrebert]

Sauce tests passed: https://travis-ci.org/twbs/bootstrap/builds/27043510