this kind of thing can happen when the gemstones are not connected correctly, try the following:
rvm get head rvm reinstall 1.9.3-p448 rvm use 1.9.3-p448
This will reinstall the ruby โโand make all the gems pristine, which should prevent such problems - unless the authors of the gems did some extra tricks that go beyond the standard ruby โโdetection.
Update
Using the bundler flag --deployment or --path not always safe, the error message says:
/Users/sashafklein/rails/<appname>/vendor/bundle/ruby/1.9.1/gems/posix-spawn-0.3.6/lib/posix_spawn_ext.bundle
This means that the stone was compiled with another ruby โโand now cannot find it.
The easiest way to fix this is to stop using the --deployment flag, as it remains, the easiest way to get rid of it (and all other potentially dangerous settings):
rm -rf .bundle/ vendor/bundle bundle install
Interpretation
The Bundler does an excellent job of executing, unfortunately, it applies another level of filtering with the --deployment flag, which forces the installation of gems on a path that is common to several rubies - in this case: ruby/1.9.1 , this unsafe since rubies can be compiled with different settings that can break the c-extension gems used in different rubies.
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