I tried installing php5-curl package on my VPS, but apt-get gave me this error message:

# apt-get install php5-curl

Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
  php5-cli php5-common php5-fpm php5-gd php5-readline
Suggested packages:
  php-pear php5-user-cache
The following NEW packages will be installed:
  php5-curl
The following packages will be upgraded:
  php5-cli php5-common php5-fpm php5-gd php5-readline
5 upgraded, 1 newly installed, 0 to remove and 156 not upgraded.
Need to get 4,956 kB of archives.
After this operation, 154 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
WARNING: The following packages cannot be authenticated!
  php5-readline php5-cli php5-fpm php5-gd php5-common php5-curl
Install these packages without verification? [y/N] y
Err http://mirrors.digitalocean.com/ubuntu/ utopic-updates/main php5-readline amd64 5.5.12+dfsg-2ubuntu4.6
  404  Not Found [IP: 95.85.0.50 80]

... (more sources)

E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/p/php5/php5-curl_5.5.12+dfsg-2ubuntu4.6_amd64.deb  404  Not Found [IP: 91.189.91.24 80]

E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

None of apt-get update or --fix-missing seems to fix this. What can I do with this issue?

Also it does the same for package curl, even when I run

# curl --version
curl 7.37.1 (x86_64-pc-linux-gnu) libcurl/7.37.1 OpenSSL/1.0.1f zlib/1.2.8 libidn/1.28 librtmp/2.3
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp smtp smtps telnet tftp
Features: AsynchDNS GSS-Negotiate IDN IPv6 Largefile NTLM NTLM_WB SSL libz TLS-SRP