Add Ruby 2.7 and 3.0 to travis-ci test matrix

[unasuke]

Summary

I want to update my own rails app to ruby 2.7. But sassc-rails is not tested on ruby 2.7 yet.
So add Ruby 2.7 and 3.0 to the travis-ci test matrix.

Changes

• Install and use bundler 1.17.3 explicitly
  • Sometimes, travis fails to use bundler 1.x
• Exclude some combinations
  • BigDecimal.new was removed.
    • https://github.com/ruby/bigdecimal/blob/master/CHANGES.md#200
• Introduced incompatible kwargs behavior from ruby 3.0