v0.5.0-alpha.2: ci: sort out our release artifacts

First of all, let's not build for every platform under the sun, it takes
a ton of time for no particularly good reason.

Second of all, let's be a little bit less verbose, we don't need to know
precisely which packages were used to compile a binary, over and over
again.

Last but not least, point travis to the actual artifacts we built. If we
build *.tar.gz, let's not try to upload all those inexistent *.tgz files
;)