Use `jpegtran` to purge EXIF data from jpegs.

XeLaTeX will still give images the wrong size when there is certain EXIF
information attached to the JPEG.  Use the `jpegtran` utility to clean
EXIF information from the JPEG before we give it to XeLaTeX.

Since the WMF test infrastructure does not have `jpegtran` installed,
skip this step when running there.

Change-Id: I1c00dc80ff052d71371300c10b16d86e98fb02ab
Merge "Use `jpegtran` to purge EXIF data from jpegs."