Fix jumbotron input-group-button offset in Internet Explorer

Using an `.input-group-button` with a `.btn-large` button in a `.jumbotron` is slightly offset in Internet Explorer. This is fixed by explicitly setting the line-height of the `.jumbotron .input-group`.

Tested in latest Firefox, Chrome and Opera -- the fix has no side-effects.
Tested in Internet Explorer 8, 9, 10 and 11 -- the fix works for all versions.

The problem and the fix can be seen in the following JSFiddle:
http://jsfiddle.net/4L6f3/