With the release of OSX Yosemite, Ruby 1.8.7 is definitely gone from the default system configuration. I decided to make a clean install of Yosemite and since then I faced an issue while trying to install ruby 1.8.7 via RVM. The problem was a dependency on gcc46 which could not be resolved.
Actualy this seems the best workaround for RVM users on Yosemite:
1 Update homebrew/homebrew-versions tap
2 Install gcc48: brew install gcc48
3 Install Ruby 1.8.7.
These steps solve the issue because RVM uses for gcc48 instead of gcc46 when gcc48 is installed.