Simplify instructions for running Ruby scripts

I helped a student with this exercise last week at Codebar. They were
using Windows, which means the sections about file permissions did not
apply and caused some confusion.

Instead of providing Windows-specific instructions I've opted to
instruct the student to use the `ruby` command directly. This means the
student doesn't have to download the empty file with shebang line
anymore.

Author: tijmenb

ruby/lesson2/tutorial.md

@@ -59,19 +59,12 @@ Random.rand(5..10)
 
 In this exercise we'll create a small game to help us practise maths.
 
-[Download the files](https://gist.github.com/despo/d790a4fd1844da7d85ce/download) to get started. If you are having any trouble you can get the files directly [from GitHub](https://gist.github.com/despo/d790a4fd1844da7d85ce).
+First, create an empty file called `numbers.rb`.
 
-
-To execute the file you must first change its permissions from the command line (to make it executable).
-
-```bash
-chmod a+x numbers.rb
-```
-
-and then run it
+To execute the file, run the following:
 
 ```bash
-./numbers.rb
+ruby numbers.rb
 ```
 
 In this exercise we want to generate two random numbers x and y using `Random.rand()`. We will output the numbers to the console and read in the answer.

ruby/lesson3/tutorial.md

@@ -53,15 +53,9 @@ end
 
 Using what we have learned in the last couple of ruby lessons we will create a contact list where we can store people's names and date of birth.
 
-Create a new ruby script file.
+Create a new ruby script file named `contacts.rb`.
 
-> Remember to change the permissions to make the file executable using `chmod a+x contacts`
-
-First, make sure you tell the script to run in a ruby environment so you can execute it with `./contacts`
-
-```ruby
-#!/usr/bin/env ruby
-```
+You can execute it using `ruby contacts.rb`.  
 
 Unlike last time, we don't want this program to exit unless we tell it to. We can do that by setting a variable to true, and only changing it to false if the user tells the program to exit.