Fix Rakefile by deleting refs to removed install tasks

Previously when replacing dependency installation with `npm` rather than
`rake`, there were still install task references found in `Rakefile`
which should have been removed as well.

```
$ npm version patch
v2.3.1

...

npm ERR! > minifying `mustache.js`...
npm ERR! rake aborted!
npm ERR! Don't know how to build task 'install_uglify'
```

Those `install_*` tasks does not exist anymore, as installing those
dependencies are done with npm now.

Refs #666

Rakefile

@@ -8,17 +8,17 @@ def minified_file
 end
 
 desc "Run all tests"
-task :test => :install_mocha do
+task :test do
   sh "./node_modules/.bin/mocha test"
 end
 
 desc "Make a compressed build in #{minified_file}"
-task :minify => :install_uglify do
+task :minify do
   sh "./node_modules/.bin/uglifyjs mustache.js > #{minified_file}"
 end
 
 desc "Run JSHint"
-task :hint => :install_jshint do
+task :hint do
   sh "./node_modules/.bin/jshint mustache.js"
 end