use explicit full path to form more informative error messages

[maxbeatty]

I've noticed quite a few issues and comments where it's hard to debug when a .env file isn't found. By using process.cwd explicitly, the full path will be included in the error message to aid in debugging. Hopefully this is an improvement from having to work backwards from a custom path to figure out what the original error message meant.

• start testing Node v9 and drop v7 since it's no longer maintained
• use process.cwd to declare a full path to read from
• remove references to iojs
• introduce some es6 syntax ref Feature: ES2015 #236

[maxbeatty]

I might suggest this be released as a MAJOR version since there's a slim possibility that some assumptions could break by changing the default path that is being read.

[coveralls]

Coverage remained the same at 100.0% when pulling b619d4d on better-err into 46ec97d on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 93df1db on better-err into 46ec97d on master.

[maxbeatty]

@motdotla will you configure AppVeyor for the repository to enable Windows testing? I think you need to do it as the repo owner. This will unblock #241 and a new release (https://github.com/motdotla/dotenv/projects/1)

[Haroenv]

this looks great! I was confused by this documentation too, since no error was thrown for non-existing files.

[maxbeatty]

I am going to merge this and release v5. #260 has been created to track adding AppVeyor