v1.6.0

• Support ImageMagick 7
• Fix memory leak with coalesce for ImageMagick
• Add sharpen, set_quality, auto_orient, get_colorspace, set_colorspace, level_image, hald_clut for GraphicsMagick
• Throw error if size string can not be parsed in thumb, handle case when source size is missing, more specs for thumb
• Update test suite to GitHub actions, remove TravisCI
  • Test suite runs for LuaJIT beta and OpenResty's LuaJIT fork
  • Currently runs on Ubuntu: ImageMagick 6.9.10, GraphicsMagick 1.3.35, Arch Linux: ImageMagick 7.0.10.61, GraphicsMagick 1.3.36
  • Fix broken spec for modulate