update to StatsBase 0.34

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (4e795e9) 95.57% compared to head (7f4141e) 95.57%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #235 +/- ## ======================================= Coverage 95.57% 95.57% ======================================= Files 8 8 Lines 610 610 ======================================= Hits 583 583 Misses 27 27 ``` | [Impacted Files](https://app.codecov.io/gh/FixedEffects/FixedEffectModels.jl/pull/235?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/FixedEffectModels.jl](https://app.codecov.io/gh/FixedEffects/FixedEffectModels.jl/pull/235?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL0ZpeGVkRWZmZWN0TW9kZWxzLmps) | `100.00% <100.00%> (ø)` | |

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