Use gpg instead of gpg2

[AsciiWolf]

Now that #412 was fixed, please also consider using "gpg" instead of "gpg2". On most Linux distributions nowadays, gpg2 is just a symlink to gpg and on some of them, for example Debian-based systems, the symlink does not exist out-of-box and so using gpg2 adds unnecessary dependency on gnupg2 which is a transitional package that just provides a symlink to gpg, is considered deprecated and will be dropped in the future. On some systems, for example the freedesktop Flatpak runtime, gpg2 is already not available at all.

/cc @micahflee

[rogers0]

@AsciiWolf this should be fixed in v0.3.4 already.