Make Memoist.escape_punctuation compatible with MRI 2.7

[casperisfine]

In https://bugs.ruby-lang.org/issues/16150 MRI merge an experimental feature that makes Symbol#to_s return a frozen string.

So unless that experiment is reverted before release, escape_punctuation need to handle string.to_s being potentially frozen.

@matthewrudy

[casperisfine]

The CI failures seem to be due to outdated travis config. Other than that it's green.

[yahonda]

Hi, I'd like this pull request reviewed because this change should address one of Rails CI failure reported at https://buildkite.com/rails/rails/builds/64100#92ed9e75-c005-42a2-af7d-64b2997f0436/984-1452

[sebjacobs]

I'm sorry for the delay but my brother, @matthewrudy, recently passed away.

According to recent activity on the original ruby issue 1: it looks like
there might be plans to revert the experiment for the time being, however I
am happy to help with getting this change merged regardless.

[sebjacobs]

I wonder if it is worth retaining these explicit .freeze calls for older versions of ruby (2.2 and below). If not then it would be good to add something to the commit message which explains the reasoning behind dropping them.

[casperisfine]

No you are right, I'll add them back.

[casperisfine]

I'm sorry for the delay but my brother, @matthewrudy, recently passed away.

I'm sorry for your loss.

[casperisfine]

As for CI failures @unasuke opened #84.

[sebjacobs]

Thanks @casperisfine. This looks good.

[casperisfine]

Thanks for the merge.

Is there anything you'd like included in the next release that I can help with?

[ghiculescu]

For those who just saw this (like me), 0.16.1 and up include this fix.