uninitialized -> undef

[saschatimme]

A little bit less deprecation warnings.

[coveralls]

Coverage remained the same at 99.017% when pulling 8a51d54 on undef into 69924f7 on master.

[codecov-io]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #12   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          15       15           
  Lines         718      718           
=======================================
  Hits          711      711           
  Misses          7        7

Impacted Files	Coverage Δ
src/diff.jl	100% <100%> (ø)	⬆️
src/mult.jl	98.36% <100%> (ø)	⬆️
src/subs.jl	98.03% <100%> (ø)	⬆️
src/poly.jl	100% <100%> (ø)	⬆️

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[blegat]

Thanks :)