Remove outdated "Note about deploying to Heroku" from docs

[brianhelba]

Heroku no longer does any special processing for the cryptography library, as it now provides wheels: heroku/heroku-buildpack-python@c373e80

[coveralls]

Coverage remained the same at 100.0% when pulling 2659a12 on brianhelba:patch-1 into 8f8cadb on orcasgit:master.

[jrenaut]

+1 to this - I'm using this on Heroku in prod without the workaround described