Fix failing packaging job.

gtksourceview depends transitively on py2cairo which was updated in
Homebrew to depend explicitly on python2 (see Homebrew/homebrew-core#24714):
this makes the python3 install step impossible.

.travis.yml

@@ -243,8 +243,8 @@ matrix:
       - EXTRA_OPAM="${LABLGTK}"
       before_install:
       - brew update
-      - brew install opam gnu-time gtk+ expat gtksourceview libxml2 gdk-pixbuf python3
-      - pip3 install macpack
+      - brew install opam gnu-time gtk+ expat gtksourceview libxml2 gdk-pixbuf python
+      - pip install macpack
       before_deploy:
         - dev/build/osx/make-macos-dmg.sh
       deploy: