utils/repology.rb: fix "String can't be coerced into Integer"

[maxim-belkin]

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brew bump errors out with:

Error: String can't be coerced into Integer
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/repology.rb:12:in `+'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/repology.rb:12:in `query_api'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/repology.rb:42:in `parse_api_response'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/bump.rb:41:in `bump'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:112:in `<main>'

because query_api expects a String and last_package_index is an Integer (see line 44)

[MikeMcQuaid]

Thanks for the quick fix!