Ubuntu 14.04 Hash Sum mismatch problem.

Some update and upgrade session might corrupt the metadata of the package information in Ubuntu, more known as the “Hash Sum mismatch” error after the  apt-get update command. A workaround if the simple update command doesn’t work is you need to manually clean the package metadata record being produced, here’s how:

1. Run these series of command on your terminal:

   apt-get clean
   rm -rf /var/cache/apt/*
   rm -rf /var/lib/apt/lists/*

2. Then check if all works out by doing another package metadata update:

   apt-get update

3. Done.

Full information about the discussion here: (https://gitlab.com/gitlab-org/omnibus-gitlab/issues/628)