A few minor changes to the box plot template.

Summary:
* Removed extra vertical whitespace
* Increased spacing between tick marks and number labels
* Made it impossible to generate an image wider than 480 pixels

Test Plan:
Changed the various number line parameters (min, max, tick size) and verified
that box plot rendered as expected.

Reviewers: jack

Differential Revision: http://phabricator.khanacademy.org/D4488