`mix phoenix.new dev_app --dev` fails inside-phoenix-directory test

[bryanstearns]

Following the instructions in the README, I attempted to generate a new project within the installer directory.

Note that this is using the old generator (which is what the README says to use); the new generator works fine (and is actually what I wanted, of course, but didn't remember to use the new name until I started tracking down this problem).

Environment

• Elixir version (elixir -v): 1.4.2
• Phoenix version (mix deps): current master, c5d5b6a
• NodeJS version (node -v): 7.6.0
• NPM version (npm -v): 4.1.2
• Operating system: macOS 10.12.3 Sierra

Expected behavior

A new project would be created.

Actual behavior

mix phoenix.new dev_app --dev failed with ** (Mix) --dev projects must be generated inside Phoenix directory.

This appears to be happening because the old generator code moved down two directory levels in this commit, but the relative path calculation when setting @phoenix in Mix.Tasks.Phoenix.New (which uses __DIR__) didn't change.

PR coming in a minute :-)