Add rexml gem into dependency for Ruby 3.0

[tricknotes]

It has been removed from default gems since Ruby 3.0.
Ref: https://www.ruby-lang.org/en/news/2020/12/25/ruby-3-0-0-released/

To use it via bundler, we should specify it as dependency.

[codecov-io]

Codecov Report

Merging #106 (ab4875e) into master (968d1a0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #106   +/-   ##
=======================================
  Coverage   97.31%   97.31%           
=======================================
  Files          23       23           
  Lines         335      335           
=======================================
  Hits          326      326           
  Misses          9        9           

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[tricknotes]

@fgrehm How do you feel about this PR?

[tricknotes]

@fgrehm Could you review this PR ?

[fgrehm]

Hey @tricknotes! sorry for the delay here, this PR is in for the next major release of the gem which I hope to release at some point this week :) https://github.com/fgrehm/letter_opener_web/compare/next

[tricknotes]

Good to know! Thank you!

[fgrehm]

Hey @tricknotes! If you have the time, it'd be great if you could give that PR above a try, tks in advance!

[fgrehm]

Just got a pre-release ready to go. Please give that a try when you get a chance and I'll also just ping some folks for testing before a final release. This PR should auto close after code is on master.

Thanks for your contribution!

[fgrehm]

Closed by #113