papers: use IPAexGothic for complex-ja CJK font (Droid Sans Japanese isn't installed on oven)
The previous setCJKmainfont was 'Droid Sans Japanese' which doesn't
exist on the oven build host. xelatex therefore fell back to Latin
Modern Roman for every CJK glyph and emitted hundreds of 'missing
character' warnings, then failed to produce a PDF.
IPAexGothic is the standard Japanese sans-serif on Ubuntu (in
fonts-ipaexfont package, already installed at
/usr/share/fonts/truetype/fonts-japanese-gothic.ttf). Switching to it
lets the build succeed.
(complex-zh.tex uses 'Droid Sans Fallback' which happens to cover
Japanese too, but ja deserves a proper Japanese font.)