Intalling redmine on an old linux centos machine was not straightforward for me, after installing ruby from source (repo had an old version and I wanted to have redmine 3.0) I got stuck in little things (missing libraries) so finally this were the steps I taken to have redmine started:

gem install bundler
yum install libxslt-devel libxml2-devel
yum install patch
yum install zlib-devel
yum install mysql-devel –nogpgcheck
wget http://www.imagemagick.org/download/linux/CentOS/x86_64/ImageMagick-6.9.0-9.x86_64.rpm
#installed imagemagick from source
gem install rails
gem install unicorn
cd web-redmine installation dir
PKG_CONFIG_PATH=/usr/local/lib/pkgconfig C_INCLUDE_PATH=/usr/local/include/ImageMagick-6 bundle install –path vendor/bundle –without development test
rake generate_secret_token
RAILS_ENV=production rake db:migrate
RAILS_ENV=production rake redmine:load_default_data
# test
ruby bin/rails server webrick -e development
# start unicorn server
unicorn_rails -p 5001 -E production -D

# configure apache as reverse proxy
<Proxy balancer://unicornservers>
BalancerMember http://127.0.0.1:5001
</Proxy>

RewriteCond %{DOCUMENT_ROOT}/%{REQUEST_FILENAME} !-f
RewriteRule ^/(.*)$ balancer://unicornservers%{REQUEST_URI} [P,QSA,L]

ProxyPass / balancer://unicornservers/
ProxyPassReverse / balancer://unicornservers/
ProxyPreserveHost on

<Proxy *>
Order deny,allow
Allow from all
</Proxy>