I believe this rvm get stableis a new command that your rvm version does not yet support. Update your version with rvm get latest, and the option get stablewill become available to you for future updates.
You can check this by simply typing rvm getto find out what your options are.
In the old version of rvm, you also need to restart:
rvm get head
rvm reload
rvm get stable
Running the installer is also safe:
bash -s stable < <(curl -s https://raw.github.com/wayneeseguin/rvm/master/binscripts/rvm-installer)
rvm reload
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