There's a two year old blog post here explaining how to fix the error 'you 'have to install development tools first' error that comes with Nokogiri, a problem that arises if you upgrade to Mountain Lion after having installed RVM. In the blog post he says you have to do
rvm update head
rvm --force install 1.9.2
gem install bundle
## in your rails app
bundle
Installing nokogiri (1.4.2) with native extensions
...
rvm update head
rvm get head
rvm --force install 1.9.2
rvm --force install 1.9.2
Searching for binary rubies, this might take some time.
No binary rubies available for: osx/10.8/x86_64/ruby-1.9.2-p320.
Continuing with compilation. Please read 'rvm mount' to get more information on binary rubies.
The provided compiler '/usr/bin/gcc' is LLVM based, it is not yet fully supported by ruby and gems, please read `rvm requirements`.
[1]: http://www.fakingfantastic.com/2010/11/26 /fixing-the-you-have-to-install-development-tools-first-error-with-nokogiri/
rvm install 1.9.2
Searching for binary rubies, this might take some time.
No binary rubies available for: osx/10.8/x86_64/ruby-1.9.2-p320.
Continuing with compilation. Please read 'rvm mount' to get more information on binary rubies.
The provided compiler '/usr/bin/gcc' is LLVM based, it is not yet fully supported by ruby and gems, please read `rvm requirements`.
you need to run:
rvm get head
rvm autolibs enable
rvm use --install 1.9.2
bundle install