fix travis-ci cache directory setting

[xuwei-k]

[codecov-io]

Codecov Report

Merging #31 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #31   +/-   ##
=======================================
  Coverage   84.69%   84.69%           
=======================================
  Files          33       33           
  Lines         464      464           
  Branches       55       55           
=======================================
  Hits          393      393           
  Misses         71       71

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update f5fd57b...0b78dde. Read the comment docs.