Update README.md

Use SVG badges instead PNG

Author: v-kolesnikov

README.md

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 # Minitest::PowerAssert
 
-[![Gem Version](https://badge.fury.io/rb/minitest-power_assert.png)](https://rubygems.org/gems/minitest-power_assert) [![Build Status](https://travis-ci.org/hsbt/minitest-power_assert.png)](https://travis-ci.org/hsbt/minitest-power_assert)
+[![Gem Version](https://badge.fury.io/rb/minitest-power_assert.svg)](https://rubygems.org/gems/minitest-power_assert)
+[![Build Status](https://travis-ci.org/hsbt/minitest-power_assert.svg)](https://travis-ci.org/hsbt/minitest-power_assert)
 
 A Power Assert implementation for Minitest. This gem is inspired by the [test-unit-power_assert](https://github.com/k-tsj/test-unit-power_assert) gem.