Install po2json from git to silence warning

[mwiencek]

Problem

"DeprecationWarning: Invalid 'main' field" has been fixed for years since https://github.com/mikeedwards/po2json/pull/97 but there was never a release.

Solution

This just installs the latest version from a git hash.

Testing

I reviewed the changelog and the diff since v0.4.1. There was one breaking change: format: 'jed1.x' is now format: 'jed'. Other than that, I didn't notice any issues compiling resources or browsing the website locally with translations enabled.

As po2json appears to be abandonware, perhaps we will have to fork it eventually, but it still works perfectly fine in the state it was left in, so I don't think there is a need yet.