Set travis and appveyor numpy version back to 'stable'

[djhoese]

The ci-helpers default for "stable" numpy version is 1.18 which is not available from conda-forge. This was causing problems in #988 but maybe now that I fixed the mitiff issue this will work (even if ci-helpers falls back to PyPI numpy).

• Tests passed

[coveralls]

Coverage decreased (-0.003%) to 89.464% when pulling fc109a3 on djhoese:ci-numpy-version into 8ffb6ec on pytroll:master.

[mraspaud]

@djhoese this is good to merge, right ?

[djhoese]

OSX is failing. I've restarted those environments.

[djhoese]

It seems like the failure is related to the pygrib, glymur, and pygac dependencies not being installed.

[codecov]

Codecov Report

Merging #1044 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1044   +/-   ##
=======================================
  Coverage   89.46%   89.46%           
=======================================
  Files         198      198           
  Lines       29157    29157           
=======================================
  Hits        26086    26086           
  Misses       3071     3071

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[djhoese]

Merged with master which already had travis on stable numpy and was passing. Now github missed the travis link so here is the latest build for this PR: https://travis-ci.org/github/pytroll/satpy/builds/666786469

[mraspaud]

Ok, good, this can be merged when it passes