Use SVG for all badges in README.md

PNG results in a pixelation on macOS when using a retina display.

README.md

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 # `gimli`
 
-[![](http://meritbadge.herokuapp.com/gimli) ![](https://img.shields.io/crates/d/gimli.png)](https://crates.io/crates/gimli)
+[![](http://meritbadge.herokuapp.com/gimli) ![](https://img.shields.io/crates/d/gimli.svg)](https://crates.io/crates/gimli)
 [![](https://docs.rs/gimli/badge.svg)](https://docs.rs/gimli/)
-[![Build Status](https://travis-ci.org/gimli-rs/gimli.png?branch=master)](https://travis-ci.org/gimli-rs/gimli)
+[![Build Status](https://travis-ci.org/gimli-rs/gimli.svg?branch=master)](https://travis-ci.org/gimli-rs/gimli)
 [![Coverage Status](https://coveralls.io/repos/github/gimli-rs/gimli/badge.svg?branch=master)](https://coveralls.io/github/gimli-rs/gimli?branch=master)
 
 A lazy, zero-copy parser for the [DWARF debugging format](http://dwarfstd.org/).