Modify ja.css for better Japanese readability

For .font-primary, change below items for better readability.

* Changed Japanese fonts from gothic to mincho.
* Changed line-height.

While <code>"ヒラギノ角ゴ Pro W3", "Hiragino Kaku Gothic Pro", Osaka, "メイリオ", Meiryo, "ＭＳ Ｐゴシック", "MS PGothic"</code> is a popular font-family set for Japanese websites, these are all gothic fonts, equivalent to sans-serif. So changed to mincho fonts, equivalent to serif fonts.

Japanese letters looks slightly bigger, compared to English letters of same font size. So larger line-height is usually specified.