more travis tweaking

try brew install rather than reinstall. The .pc files are often
created by the install method, which might explain why libpng.pc was
missing.

.travis.yml

@@ -77,7 +77,7 @@ matrix:
         - brew update
         - brew install ccache pkg-config automake
         - brew install --ignore-dependencies gtk-doc
-        - brew reinstall
+        - brew install
           gobject-introspection
           fftw mozjpeg libexif
           libpng webp libtiff