exec foreman to replace current process (#131)

* exec foreman to replace current process

Use `exec` to invoke `foreman` to make sure `bin/dev` is replaced by the `foreman` process. This ensures that `foreman` gets `bin/dev`'s pid (so it's cleaned up properly) and that interrupts and signals (like `ctrl+c`) are handled correctly by `foreman` (to shut down child processes).

* Use sh as least common denominator shell

Some distros don't ship with bash, but will have sh.

see also: rails/cssbundling-rails#98

lib/install/dev

@@ -1,8 +1,8 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
 if ! gem list foreman -i --silent; then
   echo "Installing foreman..."
   gem install foreman
 fi
 
-foreman start -f Procfile.dev "$@"
+exec foreman start -f Procfile.dev "$@"