Call PyErr_Clear() after failing to import greenstack

[AlexHill]

I haven't been able to actually test the change in the non-PyCapsule path because of issues with Python 2.6 on my system, but the PyCapsule_Import path works for me in 3.5.2 and 2.7.12 so I included it by analogy.

[tbodt]

The appropriate place to add this would be in greenstack.cpp.

[tbodt]

Also it would be a good idea to edit .travis.yml to test running v8py without greenstack installed.

[codecov-io]

Codecov Report

Merging #18 into master will increase coverage by 0.19%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   69.53%   69.72%   +0.19%     
==========================================
  Files          16       16              
  Lines        1208     1209       +1     
  Branches      190      190              
==========================================
+ Hits          840      843       +3     
+ Misses        241      240       -1     
+ Partials      127      126       -1

Impacted Files	Coverage Δ
v8py/greenstack.cpp	36.66% <100%> (+9.08%)	:arrow_up:

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[AlexHill]

Ok, I've moved the PyErr_Clear() and I'm looking at how to make Travis do what we need here.

[tbodt]

Looks good. Thanks!