Fix the broken link on Alembic

[xuhang57]

http://alembic.readthedocs.org/en/latest/tutorial.html

doesn't exist any longer.

http://alembic.readthedocs.org/en/latest/ redirects to the new site.

[coveralls]

Pull Request Test Coverage Report for Build 1716

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 59.338%

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Totals
Change from base Build 1686:	0.0%
Covered Lines:	2097
Relevant Lines:	3534

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💛 - Coveralls