README.md: add Travis badge and switch to shields.io.
README.md: add Travis badge and switch to shields.io.

[ci skip]

denji/css-minification-benchmark,denji/css-minification-benchmark,tavriaforever/css-minification-benchmark,GoalSmashers/css-minification-benchmark,tavriaforever/css-minification-benchmark,webdev1001/css-minification-benchmark,webdev1001/css-minification-benchmark,shinnn/css-minification-benchmark,shinnn/css-minification-benchmark