brew.sh: Install speedtest_cli

Hat tip: @bramus – https://www.bram.us/2014/12/17/speedtest-cli/
.osx: Kill apps with `-HUP` to make sure they restart

Note that a full system restart is needed after running `.osx` anyhow, but this small change improves the situation for those who don’t do that.

Closes #469.
.osx: Revert to plain `killall` without `-HUP`

This reverts commit 07cd2632f56be58a5c1ba41a830e537e34f24e5f.

Ref. #469.
.gitconfig: Prevent showing files whose names contain non-ASCII symbols as unversioned

http://michael-kuehnel.de/git/2014/11/21/git-mac-osx-and-german-umlaute.html

Closes #460.