3.1.0

• SVG images now use viewBox instead of width and height to scale. A new property $svgViewBoxSize has been added to QROptions, $scale has no effect. QRMarkup has a new property $svgHeader that you can overwrite when inheriting that class. This change may cause unexpected scaling results if you're using SVG output. See #30 and 090bbeb
• PHPUnit has been updated to v8.0
• minor readme, .gitignore & test fixes