Specify rmagick versions and update distribution list

For ChiliProject 3.0 we will strictly require rmagick >= 2.0. If you need it,
you might have to update your distribution or install ImageMagick from
alternative sources.

Gemfile

@@ -23,19 +23,21 @@ group :openid do
 end
 
 group :rmagick do
-  gem "rmagick"
-  # Older distributions might not have a sufficiently new ImageMagick version for
-  # the current rmagick release (current rmagick is rmagick 2, which requires
-  # ImageMagick 6.3.0 or above). If this is the case for you, comment the line
-  # above this comment block and uncomment the one underneath it to get an rmagick
-  # version known to work on older distros.
-  # The following distros (there might be more) don't ship with a new enough
-  # ImageMagick version:
-  # * Ubuntu 8.04 and older
-  # * Debian (except testing)
+  gem "rmagick" '>= 1.15.17'
+  # Older distributions might not have a sufficiently new ImageMagick version
+  # for the current rmagick release (current rmagick is rmagick 2, which
+  # requires ImageMagick 6.4.9 or later). If this is the case for you, comment
+  # the line above this comment block and uncomment the one underneath it to
+  # get an rmagick version known to work on older distributions.
+  #
+  # The following distributíons are known to *not* ship with a usable
+  # ImageMagick version. There might be additional ones.
+  #   * Ubuntu 9.10 and older
+  #   * Debian Lenny 5.0 and older
   #   * CentOS 5 and older
   #   * RedHat 5 and older
-  #gem "rmagick", "~> 1.15.17"
+  #
+  #gem "rmagick", "< 2.0.0"
 end
 
 # Use the commented pure ruby gems, if you have not the needed prerequisites on