Force rasterio version to fix PROJ4 to WKT change

[djhoese]

See pydata/xarray#2715

[coveralls]

Coverage remained the same at 77.58% when pulling 2fdc7b9 on djhoese:bugfix-rasterio-workaround into 5c5f0c5 on pytroll:master.

[codecov]

Codecov Report

Merging #595 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #595   +/-   ##
=======================================
  Coverage   77.57%   77.57%           
=======================================
  Files         136      136           
  Lines       19179    19179           
=======================================
  Hits        14879    14879           
  Misses       4300     4300

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