Fix support of unsigned types in binomial_distribution

[mborland]

Fixes: #150

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.79%. Comparing base (525a07c) to head (88d6840).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #151   +/-   ##
========================================
  Coverage    95.79%   95.79%           
========================================
  Files          116      117    +1     
  Lines         7659     7664    +5     
========================================
+ Hits          7337     7342    +5     
  Misses         322      322           

Files with missing lines	Coverage Δ
include/boost/random/binomial_distribution.hpp	97.84% <100.00%> (ø)
test/github_issue_150.cpp	100.00% <100.00%> (ø)

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