Change the install path using rvm_path instead of change rvm scripts

Actually while generating the deb package, I run a simple script to automate the process.

After download the stable `rvm`, one of the tasks is to find and replaces all occurrences of `/usr/local/rvm` by `/usr/share/rvm`.

But there is no need of it. Just inform an env var `rvm_path` with new location while running the `rvm` install script.

This will follow KISS and the rule of use `rvm` install scripts features as much as possible. 

Tasks: 

- update the generator script to not find and replace `rvm` install path;
- update `debian/postinst` to set the `rvm_path` during install.