Change push op's idx to int from string

[sanjayl]

Before submitting a PR make sure the following things have been done:

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (make test)
• The new code is not generating bytecode or M-x checkdoc warnings
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Thanks!

Please see Cider-nREPL pull #318 at:
clojure-emacs/cider-nrepl#318