Improved Windows compatibility

[Reelix]

Added code to make memory_usage Windows compatible

[coveralls]

Coverage remained the same at 100.0% when pulling d97e91b on Reelix:patch-1 into 6d240fa on wpscanteam:master.

[erwanlr]

Thanks for the PR, I was not aware of such Gem.

I've tried to use it to get the memory on MacOSX but it seems harder than I thought.

Nevertheless, there is the get_process_mem which seems to be an easy way to get the memory and it uses sys-proctable if Windows is detected.

So I've committed the changes in master and will release a new version for you to try when all specs will pass.

I will close this PR and re-open the original issue.

Thank you again!