Clean up unit test errors caused by frozen string literal

[taichi-ishitani]

I cleaned up unit test errors caused by frozen string literal.

Also I introduced CI regression test with Travis CI.
On the CI regression test, the gem is tested with following rubies:

• 1.8.7
• 1.9.2
• 1.9.3
• 2.0.0
• 2.1.0
• 2.2.0
• 2.3.0
• 2.4.0
• 2.5.0
  • with --frozen-string-literal
• 2.6.0
  • with --frozen-string-literal

Note:
Ruby 2.3 and 2.4 support frozen string literal but it seems that standard libraries for these version do not support the feature enough.
Therefore, frozen string literal is not enabled for CI regression test with Ruby 2.3 and 2.4.

[aquasync]

Looks good, thanks!

[taichi-ishitani]

Hi @aquasync ,
Thank you for merging my PR!

BTW, when will you release the latest ruby-ole gem which includes my PR?
My gem depends on the ruby-ole gem and I'm using my forked version of ruby-ole because of the frozen string literal issue.
I'd like to use the official ruby-ole gem instead of my forked version.

[aquasync]

No worries. I've released 1.2.12.2 now.

[taichi-ishitani]

Thank you !