Remove apispec from requirements

[mahenzon]

apispec is required for ApiSpec plugin (combojsonapi) and is not used in flask-combo-jsonapi directly

[coveralls]

Pull Request Test Coverage Report for Build 56

• 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 82.909%

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Totals
Change from base Build 52:	0.0%
Covered Lines:	1271
Relevant Lines:	1533

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💛 - Coveralls