Suppress internal deprecation warnings

[davidwalker]

When using integrant in a clojurescript project (with shadow-cljs) you get deprecation warnings, even though no deprecated functions are used, because of integrant's internal usage of the deprecated functions.

------ WARNING #1 - :fn-deprecated ---------------------------------------------
 Resource: integrant/core.cljc:474:4
 integrant.core/prep is deprecated
--------------------------------------------------------------------------------

------ WARNING #2 - :fn-deprecated ---------------------------------------------
 Resource: integrant/core.cljc:478:55
 integrant.core/prep-key is deprecated
--------------------------------------------------------------------------------

You also see them when running the node-tests:

~/p/integrant ❯❯❯ lein test-node
WARNING: integrant.core/prep is deprecated at line 474 /Users/david/projects/integrant/src/integrant/core.cljc
WARNING: integrant.core/prep-key is deprecated at line 478 /Users/david/projects/integrant/src/integrant/core.cljc
WARNING: integrant.core/prep is deprecated at line 236 test/integrant/core_test.cljc
WARNING: integrant.core/prep is deprecated at line 240 test/integrant/core_test.cljc
WARNING: integrant.core/prep is deprecated at line 244 test/integrant/core_test.cljc

;; ======================================================================
;; Testing with Node:


Testing integrant.core-test

Ran 20 tests containing 120 assertions.
0 failures, 0 errors.

This PR fixes only the warnings you see from core because those are the ones that affect users of integrant. I can address the warnings in core-test as well if you like.