fixed unit test errors caused by frozen string literal

[taichi-ishitani]

Hi,

I fixed unit test errors caused by frozen string literal. af6206e

To test above changes on CI regression, I also changed files below too:

• Gemfile
  • I forked and changed ruby-ole gem for frozen string literal support.
    • I open the PR for these fix.
  • When run unit tests with frozen string literal, use my forked ruby-ole gem instead of the original gem.
• Gemfile.lock
  • Removed this file because the previous Rake does not support frozen string literal
• .travis.yml
  • added newer rubies (2.3.8, 2.4.5, 2.5.3 and 2.6.1)
  • enable --enable-frozen-string-literal when ruby version is >= 2.4
    • ruby 2.3 also supports --enable-frozen-string-literal option but it seems that standard libraries does not support the feature enough.
    • Therefore, I don't combination of ruby 2.3 and --enable-frozen-string-literal option to CI regression.

If you don't want to introduce changes of Gemfile, Gemfile.lock and .travis.yml to master repository, please ignore commits 75caf13 and ec10f8b.

[zdavatz]

Aligatoo!

[taichi-ishitani]

Hi @zdavatz ,

Thank you for merging my PR ! BTW, there're something I have to tell you:

1. I disabled e-mail notification from Travis CI temporally. If you want to get e-mail notification, you need to edit .travis.yml and revert lines bellow.
   • https://github.com/zdavatz/spreadsheet/blob/077190946d24d7779c00fa0a3a890704d9a39fbc/.travis.yml#L44
2. You should edit Gemfile and revert ruby-ole dependency when my PR (aquasync/ruby-ole#24) is merged and the ruby-ole gem including my PR is released. In addition you also need to edit .travis.yml and delete USE_LATEST_RUBY_OLE=yes environment variable.
   • https://github.com/zdavatz/spreadsheet/blob/077190946d24d7779c00fa0a3a890704d9a39fbc/Gemfile#L3
   • https://github.com/zdavatz/spreadsheet/blob/077190946d24d7779c00fa0a3a890704d9a39fbc/.travis.yml#L31

[zdavatz]

1. I do not care about the Email notifications.
2. Can you please send me a pull request for this, once your PR for Ruby-Ole is merged?

[taichi-ishitani]

2. Can you please send me a pull request for this, once your PR for Ruby-Ole is merged?

OK. I will send PR when my PR for ruby-ole is merged.

[zdavatz]

Great! Thank you!