Better path handling by using --stdin, and guessing unsaved file paths

[mhaylock]

This PR is 100% inspired by @mattbrictson's comment on Issue #10.

I've taken his suggestion to use Rubocop's --stdin option and implemented the following:

• Increased the required version of Rubocop to 0.34.0 and above, as this is the first version to add support for the --stdin option.
• Where a path for the file can be determined the linter now calls rubocop with --stdin and does not use a tempfile (instead the file's current contents are passed in via STDIN).
• For new, unsaved files if a project path can be determined (based on the currently open folders for the views window) then this path is used, otherwise it falls back to the old approach of using a tempfile.

That last change means that if you create a new file it is no longer always linted with the default rubocop config, it will use the projects rubocop config (or at least those in the root of the project) if possible.

In my testing these changes mean there are no longer any issues with relative paths when used for inherit_from, Include or Exclude. Also contextual cops now work, so this fixes #10.

This will also make #27 unnecessary, and thankfully means that unsaved changes can still be linted.

[reconbot]

This is beautiful, fix up the linting issues and I'll merge and release a new version

[mhaylock]

@reconbot fixed now - sorry, I was using Sublimelinter-flake8 and Sublimelinter-pep257 and those two errors didn't show up for some reason. Running both from the command line I could see those errors, and have fixed.

[reconbot]

v2.0.0 should be hitting package control in a few hours =)

[mhaylock]

@reconbot thank you for the quick release! Saw it come through package control a while ago 😊