fix [ No such file /lib/vanity/templates/_report] when executing vanity ...

Hello Assaf,

There is an issue coming when executing the command **vanity report --output vanity.html**, It is not getting **_report**, because we have erb files and we need to tell full path with extension of file 
**/lib/vanity/templates/_report.erb**.

I have added some fix, If you can merge with your branch, may be helpful to other people :)

Thanks,