Use SVG for status badges in README

http://shields.io/ provides many standardized badges for OSS. The PNG versions provided by Shields are similar to the images currently used, but the SVG versions added here are more legible on high pixel density displays.

README.md

@@ -1,4 +1,4 @@
-# Uglifier  [![Build Status](https://secure.travis-ci.org/lautis/uglifier.png?branch=master)](http://travis-ci.org/lautis/uglifier) [![Dependency Status](https://gemnasium.com/lautis/uglifier.png?travis)](https://gemnasium.com/lautis/uglifier)
+# Uglifier  [![Build Status](https://img.shields.io/travis/lautis/uglifier/master.svg)](https://travis-ci.org/lautis/uglifier) [![Dependency Status](https://img.shields.io/gemnasium/lautis/uglifier.svg)](https://gemnasium.com/lautis/uglifier)
 
 Ruby wrapper for [UglifyJS](https://github.com/mishoo/UglifyJS2) JavaScript compressor.