chore(build): fix version script failure when called from non-base dir

Now script will work regardless of the working directory it was called
from.

This fixes problem with CMake failing when it's called from directory
other than base `qTox`.

Fix from
https://stackoverflow.com/questions/59895/getting-the-source-directory-of-a-bash-script-from-within

tools/update-versions.sh

@@ -34,14 +34,16 @@
 
 set -eu -o pipefail
 
+readonly SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+readonly BASE_DIR="$SCRIPT_DIR/../"
 
 update_windows() {
-    ( cd windows
+    ( cd "$BASE_DIR/windows"
         ./qtox-nsi-version.sh "$@" )
 }
 
 update_osx() {
-    ( cd osx
+    ( cd "$BASE_DIR/osx"
         ./update-plist-version.sh "$@" )
 }