I've been trying to fix this for a while, but I haven't been able to find a solution.
Some notes about my setup:
- I am not using VirtualBox
 - I am running Nginx 1.4.6 Ubuntu
 
The CSS's cache will not clear even when using the "expires: off;" flag in my site configuration. Here's my site config:
server {
  # Replace this port with the right one for your requirements
  listen 80;  #could also be 1.2.3.4:80
  # Multiple hostnames separated by spaces.  Replace these as well.
  server_name www.example.com; # Alternately: _
  root /var/www/;
  error_page 404 /404.html;
  location = /404.html {  
    root  /etc/nginx/errors;  
  }  
  #access_log logs/star.example.com.access.log;
  index index.php index.html index.htm;
  location ~* \.(jpg|jpeg|gif|css|png|js|ico|html)$ {
    access_log off;
    expires off;
  }
  location / {
    root /var/www;
    try_files $uri $uri/ @extensionless-php;
    index index.php index.html;
  }
  location ~ \.php$ {
    include fastcgi_params;
    fastcgi_intercept_errors on;
    # By all means use a different server for the fcgi processes if you need to
    fastcgi_pass unix:/var/run/php5-fpm.sock;
  }
  location ~ /\.ht {
    deny  all;
  }
  location @extensionless-php {
    rewrite ^(.*)$ $1.php last;
  }
}
My nginx.conf is as follows:
user www-data;
worker_processes 4;
pid /run/nginx.pid;
events {
    worker_connections 768;
    # multi_accept on;
}
http {
    ##
    # Basic Settings
    ##
    client_max_body_size 150m;
    sendfile off;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 2;
    types_hash_max_size 2048;
    # server_tokens off;
    # server_names_hash_bucket_size 64;
    # server_name_in_redirect off;
    include /etc/nginx/mime.types;
    default_type application/octet-stream;
    ##
    # Logging Settings
    ##
    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;
    ##
    # Gzip Settings
    ##
    gzip on;
    gzip_disable "msie6";
    # gzip_vary on;
    # gzip_proxied any;
    # gzip_comp_level 6;
    # gzip_buffers 16 8k;
    # gzip_http_version 1.1;
    # gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript;
    ##
    # nginx-naxsi config
    ##
    # Uncomment it if you installed nginx-naxsi
    ##
    #include /etc/nginx/naxsi_core.rules;
    ##
    # nginx-passenger config
    ##
    # Uncomment it if you installed nginx-passenger
    ##
    #passenger_root /usr;
    #passenger_ruby /usr/bin/ruby;
    ##
    # Virtual Host Configs
    ##
    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
}
Thank you :)