string sql syntax adjustment in shapelayer rendering

[jenlindner]

This commit fixes an error that occurs as a result of upgrading gdal/simpler_tiles, because gdal 2 is employing stricter quoting (strict SQL92, http://www.gdal.org/ogr_sql.html) rules.

[coveralls]

Pull Request Test Coverage Report for Build 125

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 121:	0.0%
Covered Lines:	1494
Relevant Lines:	1494

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💛 - Coveralls