replace jeweler with bundler to manage gem

Use bundler to manage the Gem.

    bundle exec rake -T
    rake build    # Build ruby-smpp-0.6.0.gem into the pkg directory.
    rake install  # Build and install ruby-smpp-0.6.0.gem into system gems.
    rake release  # Create tag v0.6.0 and build and push ruby-smpp-0.6.0.gem to Rubygems
    rake test     # Run tests