Get travis badge from master branch

[gerritholl]

Get the travis CI "tests passing" badge from the master branch rather than the develop branch. The develop branch has been inactive since May 2018.

• Closes #xxxx
• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy
• Fully documented
• Add your name to AUTHORS.md if not there already

[djhoese]

Thanks!

[coveralls]

Coverage increased (+0.01%) to 86.897% when pulling 83d7b0e on gerritholl:patch-1 into 071e34b on pytroll:master.

[codecov]

Codecov Report

Merging #1001 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1001   +/-   ##
=======================================
  Coverage   86.89%   86.89%           
=======================================
  Files         181      181           
  Lines       27497    27497           
=======================================
  Hits        23894    23894           
  Misses       3603     3603

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