Update readme (flat-square style badges)

Signed-off-by: Jack Cherng <[email protected]>

Author: jfcherng

README.md

@@ -1,12 +1,12 @@
 # php-diff
 
-<a href="https://travis-ci.org/jfcherng/php-diff"><img alt="Travis (.org) branch" src="https://img.shields.io/travis/jfcherng/php-diff/v6"></a>
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-<a href="https://packagist.org/packages/jfcherng/php-diff"><img alt="Packagist" src="https://img.shields.io/packagist/dt/jfcherng/php-diff"></a>
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+<a href="https://travis-ci.org/jfcherng/php-diff"><img alt="Travis (.org) branch" src="https://img.shields.io/travis/jfcherng/php-diff/v6?style=flat-square"></a>
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+<a href="https://packagist.org/packages/jfcherng/php-diff"><img alt="Packagist" src="https://img.shields.io/packagist/dt/jfcherng/php-diff?style=flat-square"></a>
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+<a href="https://github.com/jfcherng/php-diff/blob/v6/LICENSE"><img alt="Project license" src="https://img.shields.io/github/license/jfcherng/php-diff?style=flat-square"></a>
+<a href="https://github.com/jfcherng/php-diff/stargazers"><img alt="GitHub stars" src="https://img.shields.io/github/stars/jfcherng/php-diff?style=flat-square&logo=github"></a>
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 A comprehensive library for generating diff between two strings.
 
@@ -27,8 +27,8 @@ You may modify one from `example/diff-table.css` or write your own from zero.
 
 ## Requirements
 
-![php](https://img.shields.io/badge/php-%5E7.1.3-blue)
-![ext-iconv](https://img.shields.io/badge/ext-iconv-brightgreen)
+![php](https://img.shields.io/badge/php-%5E7.1.3-blue?style=flat-square)
+![ext-iconv](https://img.shields.io/badge/ext-iconv-brightgreen?style=flat-square)
 
 
 ## Installation