After having upgraded my installed ImageMagick with homebrew, I was in trouble to install the gem rmagick…
The only way to not lost too much time was to revert to the previous version.
Navigate to your homebrew base directory (usually this is /usr/local) Example:
Enter brew versions FORMULANAME (FORMULANAME is the formula you want to install).
You will then get something like:
1.0.1 git checkout 1234567 Library/Formula/FORMULANAME.rb 1.0.0 git checkout 0123456 Library/Formula/FORMULANAME.rb ...
Choose the desired version and check it out via copy and paste of the desired version line (leave out the version number in the beginning).
Example for getting 1.0.0:
git checkout 0123456 Library/Formula/FORMULANAME.rb
Install the formula:
brew install FORMULANAME
Then you're done!