fix(gitignore): .gitignore now generated on phoenix.new

In Elixir v0.14.1 `Path.wildcard` no longer looks up dotfiles by default
- the `match_dot` keyword is now required. Since .gitignore in the
`template` directory is a dotfile it was not being generate when
calling the `phoenix.new` task

The change was introduced in:

commit:

elixir-lang/elixir@65cf31d32db2b5896b6947b17ef135f5efefd0ee

link:

https://github.com/elixir-lang/elixir/commit/65cf31d32db2b5896b6947b17ef135f5efefd0ee