Don't fail if nltk downloads don't work

[ysgit]

hi @csurfer, thanks for your work here!

I found I had to make this change in order to be able to be able to install the package as part of a serverless app to deploy on AWS lambda. In that context it doesn't really make sense to force download the NLTK data as part of the install of your package. In fact, the install failed because it didn't have access to nlltk at build time.

[coveralls]

Coverage remained the same at 89.773% when pulling d2570b5 on ysgit:setup-without-nltk into e36116d on csurfer:master.

[coveralls]

Coverage remained the same at 89.773% when pulling d2570b5 on ysgit:setup-without-nltk into e36116d on csurfer:master.

[coveralls]

Coverage remained the same at 89.773% when pulling d2570b5 on ysgit:setup-without-nltk into e36116d on csurfer:master.

[coveralls]

Coverage remained the same at 89.773% when pulling d2570b5 on ysgit:setup-without-nltk into e36116d on csurfer:master.

[ysgit]

hey @csurfer did you get the chance to take a look at this?