- Updates test cases accordingly.
  - Adds `npm run update` script to ease updating submodules and npm
    packages after switching branches.

Fixes: OpenST#772

  - Asserts explicitly an extence of staker proxy during
    acceptStakeRequest.
  - Fixes lint issues in acceptStakeRequest unit test.
  - Removes SafeMath as its not needed after removing
    activeStakeRequestCount.

Adds a test case to test a failure of OSTComposer::acceptStakeRequest
in case if a corresponding staker proxy does not exist.