Use "gj" and "gk" to fix navigation in vim mode

As per the vim mode author's suggestion (see
https://github.com/marijnh/CodeMirror/pull/1743#issuecomment-22989545
), remap "j" and "Down" to "gj", "k" and "Up" to "gk". This is a
common practice among vim users and only makes a trivial difference
on wrapped lines.

Removed inferior navigational hacks.

Change-Id: I5a089b8ddc1237f3f93dd30c98858896a53f9111