On newer computers, RVM may have trouble building older Rubies. E.g., rvm 1.26 cannot build ruby-1.9 on Mac OS X 10.10 Yosemite.
However, by adding a compiler flag, we can successfully build and use these old Rubies:
$ rvm install 1.9 --with-gcc=clang
...
$ rvm use 1.9
$ rvm install 1.8 --with-gcc=clang
...
$ rvm use 1.8