Improve build system.

- Exclude rake from Gemfile. Just like other software don't declare a
  dependency on make. rake comes with Ruby 2.x by default, which is
  all we need.
- Renamed 'install' task to 'deps'. 'deps' task itself is composed of
  sub-tasks which can be invoked individually. Rake tries to remember
  which sub-tasks of 'deps' has been executed. So if fetching deps
  fails at any point, it can be resumed from the failed step.
- It was often needed to run rake twice after cloning the repository.
  Fixed that.
- Make configure task friendly to docker.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>