Add support for nrepl 0.6.x

[cichli]

• Drop support for clojure.tools.nrepl
• Add support for nrepl 0.6.x; use new session exec
• Test against nrepl 0.4, 0.5, 0.6

[codecov-io]

Codecov Report

Merging #100 into master will increase coverage by 0.92%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   84.49%   85.41%   +0.92%     
==========================================
  Files           1        1              
  Lines         187      192       +5     
  Branches        9       10       +1     
==========================================
+ Hits          158      164       +6     
+ Misses         20       19       -1     
  Partials        9        9

Impacted Files	Coverage Δ
src/cider/piggieback.clj	85.41% <100%> (+0.92%)	:arrow_up:

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[bbatsov]

LGMT 👍