Fix some deprecated IO.readline/0 calls.

config.exs

@@ -6,7 +6,7 @@ IO.puts("Some variables (such as FLECT_CC_ARGS and FLECT_LD_ARGS) can be left em
 IO.puts("")
 
 IO.puts("Press Enter to start configuring Flect.")
-IO.readline()
+IO.read(:stdio, :line)
 
 target = :unicode.characters_to_binary(:erlang.system_info(:system_architecture))
 
@@ -200,7 +200,7 @@ IO.puts("    FLECT_SH_LIB_DIR = #{sh_lib_dir}")
 IO.puts("")
 
 IO.puts("Press Enter to write the configuration.")
-IO.readline()
+IO.read(:stdio, :line)
 
 cfg = "# Generated by config.exs on #{inspect(:erlang.localtime())}.