Fix AppImage generation

[probonopd]

Once it is correctly uploading using provider: releases, you can remove the line using transfer.sh.

Note that I get

go build -o tumblr-downloader
mv -fv tumblr-downloader tumblr-downloader-gui/
make -C tumblr-downloader-gui/ -f Makefile.meta 
mv: cannot stat ‘tumblr-downloader’: No such file or directory
make: *** [prepare-gui] Error 1

and the Travis CI status is red, but apparently this seems unrelated to my patch and the AppImage get generated anyway.

[neuro-sys]

@probonopd

First of all, thanks for the PR!

I've just learnt that the parallelizing -j flag in make -j4 breaks the build, somehow causing a race condition, and executing a rule before the previous one has finished, thus resulting in a file not found.

So if you remove -j4 (and also one of the other make commands, because there are two), it should not give an error.

[probonopd]

Thanks for finding this out, build passed. Don't forget to remove the line using transfer.sh and use the GitHub Releases provider instead if you like.

[neuro-sys]

Could you please squash your commits to remove the extraneous ones? Then I can merge this, and I will modify it in a separate PR to make deploy: releases work.

[probonopd]

Please enable this GitHub functionality on your repo. It allows you to squash the commits when merging.