Bump cairosvg from 2.1.3 to 2.5.1 in /backend

Bumps [cairosvg](https://github.com/Kozea/CairoSVG) from 2.1.3 to 2.5.1.
- [Release notes](https://github.com/Kozea/CairoSVG/releases)
- [Changelog](https://github.com/Kozea/CairoSVG/blob/master/NEWS.rst)
- [Commits](Kozea/CairoSVG@2.1.3...2.5.1)

Signed-off-by: dependabot[bot] <support@github.com>

backend/requirements.txt

@@ -4,7 +4,7 @@ blinker==1.4
 CacheControl==0.12.4
 cachetools==2.1.0
 cairocffi==0.8.0
-CairoSVG==2.1.3
+CairoSVG==2.5.1
 certifi==2018.4.16
 cffi==1.11.5
 chardet==3.0.4