fix maxBucketSize

[zhiqiangxu]

This PR tries to make maxBucketSize more close to (log n) + 1.

i.e, it returns 1 for size 1, returns 2 for size 2,3, returns 3 for size 4,5,6,7 etc.

[codecov-io]

Codecov Report

Merging #41 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #41   +/-   ##
=======================================
  Coverage   85.43%   85.43%           
=======================================
  Files           7        7           
  Lines         728      728           
=======================================
  Hits          622      622           
  Misses         78       78           
  Partials       28       28

Impacted Files	Coverage Δ
rank_pairing/rank_pairing_heap.go	83.73% <100%> (ø)	:arrow_up:

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[zhiqiangxu]

@theodesp Could you help verify this PR?