Update node version for Travis

[michaelmior]

The current version specified is quite old and is causing tests to fail.

[codecov-commenter]

Codecov Report

Merging #138 (31ae36b) into master (0cb224b) will decrease coverage by 0.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
- Coverage   87.14%   86.63%   -0.52%     
==========================================
  Files          21       21              
  Lines         459      561     +102     
  Branches      112      148      +36     
==========================================
+ Hits          400      486      +86     
- Misses         53       69      +16     
  Partials        6        6              

Impacted Files	Coverage Δ
src/visitors/Code.js	54.34% <0.00%> (-8.16%)	⬇️
src/visitors/Conditional.js	96.00% <0.00%> (-4.00%)	⬇️
src/utils/parse.js	88.88% <0.00%> (-2.42%)	⬇️
src/visitors/Case.js	76.00% <0.00%> (-1.28%)	⬇️
src/visitors/Tag.js	87.50% <0.00%> (-1.03%)	⬇️
src/utils/parse-expression.js	76.00% <0.00%> (-0.20%)	⬇️
src/index.js	100.00% <0.00%> (ø)
src/parse-pug.js	100.00% <0.00%> (ø)
src/utils/jsx.js	100.00% <0.00%> (ø)
src/utils/interpolation.js	100.00% <0.00%> (ø)
... and 11 more

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