Sylpheed 3.8.0beta1-6

Resolve all known issues, update dependencies:

• Fixed Darwin arch and version in X-Mailer header (#9)
• Fixed copy-paste hotkeys for some selectable labels (#10)
• Updated dependencies (autoconf, cairo, enchant, gettext, glib, gpgme, harfbuzz, libassuan, libjpeg, libpng, libtiff, libxml2, libxslt, oniguruma, openssl, pango, pixman, sqlite, xz)
• Dropped support for OS X 10.9 (Mavericks) due to recent cairo incompatibility