Fix inkscape install errors on Travis
Restored inkscape installing on travis-ci.
Merge pull request #1890 from pelson/travis_inkscape

Restored inkscape installing on travis-ci.
Fix #1894: Composite images do not have a transparent background
Merge pull request #1894 from mdboom/image/composite_background

Fixes background coloring of multiple images on the same axes.
Merge remote-tracking branch 'upstream/v1.2.x'