Remove support of Foreman altogether

In order to support it we needed to patch Foreman. However, the Foreman
author rejected it.

Details: https://github.com/ddollar/foreman/pull/536

lib/pry/repl.rb

@@ -225,12 +225,6 @@ class Pry
       return if @readline_output
       if piping?
         @readline_output = (Readline.output = Pry.config.output)
-
-        # Foreman fix. Without this the output is slightly contaminated, because
-        # when Readline initialises, it overwrites current line (which already
-        # has some text from Foreman). One side effect is that it works every
-        # time you pipe output.
-        @readline_output.puts
       end
     end
   end

spec/Procfile

@@ -1,3 +0,0 @@
-# Run this with:
-# gem install foreman && foreman start -f spec/Procfile
-pryhere: sh -c '(cd ..; rake pry)'