Allow overriding the install url.

I am behind a proxy that for some reason does not guess that 'get.rvm.io' should
be redirected to 'https://get.rvm.io'. With this solution I can explicitely
declare the https url.
Another solution would be to hard code the protocol "https://" within the
variable command_fetch.
Fix default rvm url
Merge pull request #90 from real-chocopanda/allow-choosing-http-protocol

Allow overriding the install url.