Enable Travis tests on Julia 1.3

[mohamed82008]

This PR enables Travis tests on Julia 1.3.

[codecov]

Codecov Report

Merging #22 into master will increase coverage by 4.4%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #22     +/-   ##
=========================================
+ Coverage   84.19%   88.59%   +4.4%     
=========================================
  Files           8        9      +1     
  Lines         658      728     +70     
=========================================
+ Hits          554      645     +91     
+ Misses        104       83     -21

Impacted Files	Coverage Δ
src/DynamicPPL.jl	100% <0%> (ø)
src/compiler.jl	93.85% <0%> (+1%)	⬆️
src/prob_macro.jl	93.69% <0%> (+1.77%)	⬆️
src/varinfo.jl	87.72% <0%> (+5.52%)	⬆️
src/utils.jl	45.45% <0%> (+18.18%)	⬆️
src/contexts.jl	83.33% <0%> (+33.33%)	⬆️

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