Enable bundle

`slim` 1.3.8 requires `tilt` ~> 1.3.3, this butts against another dependancies with `tilt`,
(this is `bundler` bug???) so update `slim` obviously.

Another (temporary) solution:
```
diff --git a/Gemfile b/Gemfile
index a615d4e..04691bb 100644
--- a/Gemfile
+++ b/Gemfile
@@ -30,6 +30,7 @@ group :development do
   gem "haml",      ">= 2.2.22"
   gem "erubis",    ">= 2.7.0"
   gem "slim",      ">= 1.3.0"
+  gem "tilt",      "~> 1.3.3"
```