Fix geotiff and png writers to save to a temporary directory

[djhoese]

This was my mistake when I originally created these tests that the base_dir should have been specified in the writer init instead of the save_datasets method. This also adds bdist_wheel to the travis deployment. We'll see how that works next time we tag a release.

• Tests added
• Tests passed
• Passes git diff origin/develop **/*py | flake8 --diff

[coveralls]

Coverage decreased (-0.2%) to 63.888% when pulling 660937e on davidh-ssec:bugfix-tif-tests into 2dbb93a on pytroll:develop.

[codecov-io]

Codecov Report

Merging #257 into develop will decrease coverage by 0.18%.
The diff coverage is 92.59%.

@@             Coverage Diff             @@
##           develop     #257      +/-   ##
===========================================
- Coverage    64.06%   63.88%   -0.19%     
===========================================
  Files          112      112              
  Lines        13863    13879      +16     
===========================================
- Hits          8882     8867      -15     
- Misses        4981     5012      +31

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_geotiff.py	95% <100%> (+0.66%)	⬆️
satpy/tests/writer_tests/test_simple_image.py	93.75% <81.81%> (-3.69%)	⬇️
satpy/writers/geotiff.py	38.88% <0%> (-33.34%)	⬇️
satpy/utils.py	57.94% <0%> (-1.87%)	⬇️
satpy/writers/__init__.py	73.85% <0%> (+3.18%)	⬆️

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