Rework the menu

* Much more compact
* Loads the metrics for an entity the first time you click the entity
* Marks in the menu the current graphs
Fix the metrics sorting in menu
Fix clicks on menu
Bump gem dependancies

Includes :
* Latest rails version, with security fixes
* Latest bootstrap/fontawesome
* HAML 4
Merge pull request #27 from renchap/bump_gems

Bump gem dependancies
Display current graphs as active in the menu when loaded
Merge pull request #26 from renchap/rework_menu

Rework the menu
Forgotten a fontawesome variable