Make Railties CI log for Ruby 2.6 accessible again

Pending the next release of Thor which [fixes](https://github.com/erikhuda/thor/commit/006832ea32480618791f89bb7d9e67b22fc814b9) calls to `ERB.new`, Railties CI log for Ruby 2.6 is flooded with so many warnings it is too long for Travis to handle:

```
/home/travis/.rvm/gems/ruby-head/gems/thor-0.20.0/lib/thor/actions/file_manipulation.rb:120: warning: Passing safe_level with the 2nd argument of ERB.new is deprecated. Do not use it, and specify other arguments as keyword arguments.
/home/travis/.rvm/gems/ruby-head/gems/thor-0.20.0/lib/thor/actions/file_manipulation.rb:120: warning: Passing trim_mode with the 3rd argument of ERB.new is deprecated. Use keyword argument like ERB.new(str, trim_mode: ...) instead.
/home/travis/.rvm/gems/ruby-head/gems/thor-0.20.0/lib/thor/actions/file_manipulation.rb:120: warning: Passing eoutvar with the 4th argument of ERB.new is deprecated. Use keyword argument like ERB.new(str, eoutvar: ...) instead.

The log length has exceeded the limit of 4 MB (this usually means that the test suite is raising the same exception over and over).

The job has been terminated
```

https://travis-ci.org/rails/rails/jobs/372623604#L10000
https://api.travis-ci.org/v3/job/372623604/log.txt

This patch forces installation of fixed Thor, and enables us to look at the the log.

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