Readme: replace PNG badge with SVG one

README.md

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 Bean Validation / Collection Validators
 =======================================
 [![Build Status](https://travis-ci.org/jirutka/validator-collection.svg?branch=master)](https://travis-ci.org/jirutka/validator-collection)
-[![Coverage Status](https://coveralls.io/repos/jirutka/validator-collection/badge.png?branch=master)](https://coveralls.io/r/jirutka/validator-collection?branch=master)
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/cz.jirutka.validator/validator-collection/badge.svg)](https://maven-badges.herokuapp.com/maven-central/cz.jirutka.validator/validator-collection)
+[![Coverage Status](https://img.shields.io/coveralls/jirutka/validator-collection/master.svg?style=flat)](https://coveralls.io/r/jirutka/validator-collection?branch=master)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/cz.jirutka.validator/validator-collection/badge.svg?style=flat)](https://maven-badges.herokuapp.com/maven-central/cz.jirutka.validator/validator-collection)
 
 Neither [Bean Validation 1.1][JSR-349] (JSR 303/349) nor [Hibernate Validator], the reference _(and the only one…)_
 implementation of it, provide simple way to validate a collection of basic types like String, Integer, Date… (i.e.