Problem installing rmagick gem on Heroku

I'm trying to push a Rails app to Heroku, but it's failing when it tries to build the rmagick gem (which is specified in the Gemfile). I've done bundle install locally and the app runs fine.

But, on Heroku in the pre-receive hook, after it build the rmagick examples, I get:

make install

/usr/ruby1.8.7/bin/ruby setup.rb install
rm -f InstalledFiles
---> lib
mkdir -p /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib
install RMagick.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/
---> lib/rvg
mkdir -p /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install embellishable.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install pathdata.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install stylable.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install describable.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install deep_equal.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install misc.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install text.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install rvg.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install container.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install stretchable.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install paint.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install clippath.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install units.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
install transformable.rb /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/rvg
<--- lib/rvg
<--- lib
---> ext
---> ext/RMagick
mkdir -p /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib
install RMagick.so /disk1/tmp/build_3eu8mw7r34j0f/.bundle/gems/ruby/1.8/gems/rmagick-1.15.17/lib/.
<--- ext/RMagick
<--- ext
post-install.rb: installing documentation...
mkdir /usr/ruby1.8.7/share/RMagick
hook /disk1/tmp/build_3eu8mw7r34j0f/.bundle/开发者_Python百科gems/ruby/1.8/gems/rmagick-1.15.17/./post-install.rb failed:
Permission denied - /usr/ruby1.8.7/share/RMagick
Try 'ruby setup.rb --help' for detailed usage.
make: *** [install] Error 1

Then it bails out of the push. I'm using the bamboo-ree-1.8.7 stack. Do I need to add something extra to get it to work? Alternatively, it seems that it's only a documentation problem. I don't need the documentation on Heroku, so can I disable that instead?

Thanks in advance.

You Gemfile entry should be:

gem "rmagick", :require => 'RMagick'