Move mix.lock to .gitignore

[nappex]

Currently, dependencies update by $ mix deps.update change mix.lock file. Particulary, hashes of dependencies that are created by its version.
I think mix.lock should be generated locally by run commands as $ mix deps.update, it should not be created or rewritten by git pull.

[Glutexo]

The general consensus is to commit the lock file: https://duckduckgo.com/?q=should+I+commit+gemfile.lock The reasoning is that the lock file provides an environment tested to be working and that tests can be run on. If you want to bump dependencies, you can always delete the lock file locally and run deps.get yourself. The lock file provides additional information and you are free to use it or not.