Update for Nim 2.x. No longer building pit_api.

`pit_api` cannot be built for Nim 2.x without a non-trivial refactor due
to the enforcement of gc-safety. See the `pit_api.nim` source for
details.

src/pit_api.nim

@@ -1,5 +1,11 @@
 ## Personal Issue Tracker API Interface
 ## ====================================
+#
+# **NOTE** This is currently not being built as it no longer works under Nim
+# 2.x due to the inability to call system calls (invoke pit via cli) in a
+# gc-safe manner. It should be rewritten to use the functionality exposed by
+# libpit directly rather than calling the pit cli executable. Unfortunately
+# this would require a non-trivial rewrite.
 
 import asyncdispatch, cliutils, docopt, jester, json, logging, options, sequtils, strutils
 import nre except toSeq