Test suite failures for gamma distribution

[lorinder]

I occasionally see the testsuite fail for the gamma distribution.

log density check:                                                                     FAIL    
  *** Failed! Falsified (after 54 tests and 2 shrinks):
  improperGammaDistr 80.10044478990355 0.5569517767725765
  36.1
  density    = 1.850883361798215e-2
  logDensity = -3.989507167997367
  log p      = -3.9895071679973992
  ulps[log]  = 73
  ulps[lin]  = 172
  Use --quickcheck-replay=158071 to reproduce.
  Use -p '/Tests for: GammaDistribution.log density check/' to rerun this test only.

To reproduce, run the test suite in a loop. It takes typically a few dozen runs to see this.