Don't look in node_modules

[jackfranklin]

gulp-elm for example has a Test.elm file that it tries to upgrade, which isn't needed. I'd suggest just avoiding that directory entirely.

[jackfranklin]

I'm happy to make this PR too btw, just raising it now while I remember! If no one beats me to it I can do it tomorrow.

[avh4]

elm-upgrade only runs elm-format on folders that are listed as source-directories in the elm-package.json. However, if . is listed, then elm-format . will find files in node_modules. I think this issue should be moved to elm-format: https://github.com/avh4/elm-format/issues

[jackfranklin]

Ah, I'll open an issue there. I think that my Haskell knowledge (or lack of) will prevent me doing the PR unfortunately. I'll close this.