Remove additional install of pkg-config, automake and giflib from macOS CI

The Travis build for macOS is failing because brew errors out when it
finds pkg-config, automake and giflib have already been installed.

.travis.yml

@@ -71,7 +71,7 @@ matrix:
       before_install:
         - brew tap homebrew/science
         - brew update
-        - brew install ccache pkg-config automake
+        - brew install ccache
         - brew install --ignore-dependencies gtk-doc
         - brew install
           gobject-introspection
@@ -81,5 +81,5 @@ matrix:
           cfitsio libmatio
           orc little-cms2 poppler
           pango libgsf openslide
-          librsvg giflib openexr
+          librsvg openexr
         - brew install pygobject3