Switch coverage badge from develop to main branch

[ogenstad]

This is to reflect the current default branch on GitHub where the develop branch no longer exists.

Note, due to #935 this will cause the reported value to drop until that issue is fixed.

[coveralls]

Pull Request Test Coverage Report for Build 9902968351

Details

• 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 56.677%

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Totals
Change from base Build 9893690876:	0.0%
Covered Lines:	539
Relevant Lines:	951

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💛 - Coveralls