Drop Missings and use Base.nonmissingtype

[kleinschmidt]

Fixes #150

Also updates the travis script to test on 1.0, 1.2, 1.3, and nightly, dropping
1.1; also don't test on OSX anymore

[codecov-io]

Codecov Report

Merging #151 into master will decrease coverage by 0.43%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #151      +/-   ##
=========================================
- Coverage   84.04%   83.6%   -0.44%     
=========================================
  Files           9       9              
  Lines         495     494       -1     
=========================================
- Hits          416     413       -3     
- Misses         79      81       +2

Impacted Files	Coverage Δ
src/modelframe.jl	88.63% <ø> (ø)	⬆️
src/StatsModels.jl	100% <ø> (ø)	⬆️
src/terms.jl	84.34% <0%> (-1%)	⬇️
src/schema.jl	80.9% <0%> (-0.91%)	⬇️

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