Fixes .travis.yml: support latest ruby versions

[kurotaky]

Thank you for making useful holiday_jp gem.
I add latest ruby version(2.1.5, 2.2.2) to .travis.yml.
test was passed.

benchmark

I think there is no perfomance ploblem.

ruby 1.9.3p551 (2014-11-13 revision 48407) [x86_64-darwin14.3.0]

ruby benchmark.rb
  0.100000   0.010000   0.110000 (  0.108696)
  5.440000   0.040000   5.480000 (  5.601141)

ruby 2.1.5p273 (2014-11-13 revision 48405) [x86_64-darwin13.0]

ruby benchmark.rb
  0.040000   0.010000   0.050000 (  0.057441)
  5.070000   0.040000   5.110000 (  5.186538)

ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-darwin13]

ruby benchmark.rb
  0.040000   0.000000   0.040000 (  0.046594)
  5.040000   0.030000   5.070000 (  5.134255)

[komagata]

@kurotaky ⭐ 🌟 ⭐ 🌟 ⭐

[kurotaky]

😄 ⭐