Fix field name in channel.subscription.message payload

[ashkeel]

According to twitch docs, the cumulative months field in channel.subscription.message is cumulative_months, not cumulative_total

[coveralls]

Pull Request Test Coverage Report for Build 1544220163

• 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 92.523%

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Totals
Change from base Build 1364869930:	0.0%
Covered Lines:	1225
Relevant Lines:	1324

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💛 - Coveralls