fix: use Crystal 1.7.3

[mrexox]

Closes https://github.com/coverallsapp/coverage-reporter/issues/65

:zap: Summary

In Crystal 1.8 they use pcre2 (PR), and it raises an exception when parsing non UTF-8 text. It's hard to find out what exactly was wrong (as said here, PCRE2 devs don't remember that :thinking:), so let's leave it for the later work. This PR downgrades Crystal version where everything worked just fine.

:ballot_box_with_check: Checklist

• [x] Add specs

[coveralls-official[bot]]

Pull Request Test Coverage Report for Build 4817258498

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.566%

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Totals
Change from base Build 4809347986:	0.0%
Covered Lines:	720
Relevant Lines:	795

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💛 - Coveralls