Integrate ENV variables for reading PATH to credential JSON file

[khusnetdinov]

Given:
Google Translate class gets the path to JSON file via hardcoded string 'env.json'
https://github.com/symbiod/localizator/blob/master/lib/localizator/translation.rb#L11

Need:

• Pass path to JSON file via 'ENV['CREDENTIALS']'
• Create .env.example file with empty variable
• Add to gitignore .env file, that is used for local developemnt
• Update CHANGELOG.md