cmake is required during the installation of the rugged gem

The following error is encountered during the installation of the rugged gem:

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /usr/bin/ruby2.1 extconf.rb
    checking for cmake... no
    ERROR: CMake is required to build Rugged.
    *** extconf.rb failed ***
Merge pull request #7540 from sameersbn/docs-install-cmake

cmake is required during the installation of the rugged gem