Add some version/env sanity checks to 'arc'
Add some version/env sanity checks to 'arc'

Summary:
A user filed an issue on GitHub about not having 'json_decode()':

  https://github.com/facebook/arcanist/issues/10

Detect this issue and raise a detailed error message.

Test Plan:
  - Set minimum version to 6.0.0, arc told me to upgrade.
  - Changed function/flag configuration and hit the flag and no-flag error
pathways.

Reviewers: btrahan, jungejason, rm

Reviewed By: rm

CC: aran, rm, epriestley

Differential Revision: https://secure.phabricator.com/D1267

jra3/arcanist,memsql/arcanist,Symplicity/arcanist,codelabs-ch/arcanist,haskell-infra/arcanist,calblueprint/arcanist,calblueprint/arcanist,folsom-labs/arcanist,eSpark/arcanist,KieranHayes/arcanist,folsom-labs/arcanist,hach-que/arcanist,Symplicity/arcanist,r4nt/arcanist,Khan/arcanist,jra3/arcanist,wikimedia/phabricator-arcanist,coursera/arcanist,dguo-coursera/arcanist,r4nt/arcanist,Symplicity/arcanist,wikimedia/phabricator-arcanist,wikimedia/phabricator-arcanist,wharris/arcanist,hach-que/arcanist,julkiewicz/arcanist,KieranHayes/arcanist,eSpark/arcanist,wxstars/arcanist,wxstars/arcanist,chundongwang/arcanist,aik099/arcanist,Khan/arcanist,aik099/arcanist,coursera/arcanist,kevinseelbach/arcanist,wharris/arcanist,codelabs-ch/arcanist,calblueprint/arcanist,zhihu/arcanist,dguo-coursera/arcanist,fivestars/arcanist,calblueprint/arcanist,r4nt/arcanist,codelabs-ch/arcanist,alexmv/arcanist,hach-que/arcanist,phacility/arcanist,PageUpPeopleOrg/arcanist-patches,folsom-labs/arcanist,sneakpeeq/arcanist,codelabs-ch/arcanist,eSpark/arcanist,uber/arcanist,PageUpPeopleOrg/arcanist-patches,Khan/arcanist,julkiewicz/arcanist,zhihu/arcanist,fivestars/arcanist,sneakpeeq/arcanist,jra3/arcanist,alexmv/arcanist,phacility/arcanist,Khan/arcanist,dguo-coursera/arcanist,uber/arcanist,jra3/arcanist,sneakpeeq/arcanist,julkiewicz/arcanist,eSpark/arcanist,coursera/arcanist,zhihu/arcanist,haskell-infra/arcanist,folsom-labs/arcanist,memsql/arcanist,wharris/arcanist,phacility/arcanist,coursera/arcanist,memsql/arcanist,codelabs-ch/arcanist,chundongwang/arcanist,wikimedia/phabricator-arcanist,zhihu/arcanist,PageUpPeopleOrg/arcanist-patches,aik099/arcanist,phacility/arcanist,dguo-coursera/arcanist,memsql/arcanist,sneakpeeq/arcanist,chundongwang/arcanist,hach-que/arcanist,kevinseelbach/arcanist,haskell-infra/arcanist,alexmv/arcanist,Symplicity/arcanist,alexmv/arcanist,r4nt/arcanist,phacility/arcanist,KieranHayes/arcanist,aik099/arcanist,PageUpPeopleOrg/arcanist-patches,julkiewicz/arcanist,uber/arcanist,fivestars/arcanist,fivestars/arcanist,kevinseelbach/arcanist,wharris/arcanist,wxstars/arcanist,chundongwang/arcanist,kevinseelbach/arcanist,haskell-infra/arcanist,wxstars/arcanist,uber/arcanist,KieranHayes/arcanist