Tunnelblick 3.4.2 (and other changes)

Tunnelblick 3.4.2 was released today.  I've made changes to the cask file that:
 * update to the latest version;
 * specify the supported Mac OS X versions;
 * specify the GPL; and
 * add an uninstall stanza to remove a launchctl item and quit the app if running.

I don't really like the verbose version number.  I was tempted to change it to SemVer.org style
("3.4.2+4055.4161") or just omit the build specifier completely.
Please remove this line. Stanzas are organised in a particular order, and `url` and `appcast` should be together.