Bump requests version for security reasons

Author: drewbo

requirements.txt

@@ -14,7 +14,7 @@ pyclipper==1.0.6
 pycurl==7.43.0.1
 pyproj==1.9.5.1
 rasterio==1.0a12
-requests==2.11.0
+requests>=2.20.0
 Shapely>=1.6.3
 six==1.10.0
 tilepie==0.2.1