Include license file in source tarball

[CurtLH]

The LICENSE is currently not included in the source tarball.

[CurtLH]

I'm not sure why the test failed, but it doesn't appear to be related to the changes I'm proposing.

Link to error

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ERROR: Check that the R notebook generated from Rmd looks the same
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Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.6.5/lib/python3.6/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/home/travis/build/aaren/notedown/tests.py", line 396, in test_R
    knitr = notedown.Knitr()
  File "/home/travis/build/aaren/notedown/notedown/notedown.py", line 600, in __init__
    raise self.KnitrError(message)
notedown.notedown.Knitr.KnitrError: Could not load knitr (needs manual installation).
$ Rscript -e require(knitr)
Loading required package: knitr
Warning message:
In library(package, lib.loc = lib.loc, character.only = TRUE, logical.return = TRUE,  :
  there is no package called ‘knitr’
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