Ensure that RUBYGEMS_HOST is defined  

Hello, 

Empty string are evaluated to true so this patch avoid this
 ~~~
Enter your  credentials. 
Don't have an account yet? Create one at https:///sign_up
 ~~~

I don't know how I could add a test for this but let me know if I should add it.

I've added an entry in the `History.txt` file, I don't know if it's the good place for it.

Have a nice day.