Improve readme code blocks

Author: derTobsch

README.md

@@ -6,13 +6,13 @@ Compresses the css output from less using [clean-css](https://github.com/jakubpa
 
 ## lessc usage
 
-```
+```bash
 npm install -g less-plugin-clean-css
 ```
 
 and then on the command line,
 
-```
+```bash
 lessc file.less --clean-css="--s1 --advanced --compatibility=ie8"
 ```
 

lib/clean-css-processor.js

@@ -43,7 +43,7 @@ module.exports = function() {
 
             if (sourceMap) {
                 if (sourcesContent) {
-                    for (var source = 0; source < sources.length; source++) {
+                    for (let source = 0; source < sources.length; source++) {
                         output.sourceMap.setSourceContent(sources[source], sourcesContent[source]);
                     }
                 }