Fix CI badges in README

[djhoese]

They were pointing to develop instead of master. I made this PR instead of pushing directly to master because of hacktoberfest. 😜

[coveralls]

Coverage remained the same at 73.078% when pulling b7c1ed8 on djhoese:bugfix-readme-badges into 4e1e149 on pytroll:master.

[codecov]

Codecov Report

Merging #440 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #440   +/-   ##
=======================================
  Coverage   73.07%   73.07%           
=======================================
  Files         134      134           
  Lines       17662    17662           
=======================================
  Hits        12907    12907           
  Misses       4755     4755

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