Improve the coverage report

[Shekharrajak]

When we run the rspec; using simpleCov we get the coverage report . Try to improve it as much as possible.

We just need more test cases written in spec folder for all the adapters and files.

[Shekharrajak]

run bundle exec rspec and you can see the coverage report at daru-view/coverage/index.html . By clicking on the files, we can see the lines to be tested.

[Shekharrajak]

coveralls gem is added to keep track the coverage report. We can check the coverage here :
https://coveralls.io/github/SciRuby/daru-view?branch=master

Need to write specs for not covered lines .