Use Global Gem Install

Durring the setup the "sass" gem is installed. This works fine on Linux operating systems like Ubuntu. However, on some other distrobutions such as Arch Linux "gem install" defaults to the a local folder in the users home directory. When installed here the sass binary is not on the path and coco-brunch cannot find it (and thus crashes with an error). By adding the "--no-user-install" flag we are telling gem to install it globally no matter what the system default is thus fixing this problem.
Update Mongo

This updates the installer script to use the latest mongo (v2.6 nightly)
Fix Python Download

bootstrap.sh was using the "open" command to open a link. While this may work on a Mac, the command does not exist on Linux. Instead we can now use a switch to use the correct command for the users OS.

Ohh and python.org/getit is a 404 -> Changed it to python.org/download
Fix Mac Download

I had a copy-paste error when changing the mongo download links in my previous commit. This fixes that.

https://github.com/smasher816/codecombat/commit/98adf2fe4f09419040e1ee839388283ff3a47eb6
Update Completion Message

Adds a link to the "MongoDB database import" section of the wiki, as this step must be completed before they can play any levels.

Also adds some whitespace :)
Add Help Message

Not everyone runs Ubuntu/Debian and is using apt-get, so add a friendly reminder to check with your distro's documentation for help.
Merge pull request #733 from smasher816/master

Installer Changes