Fix missing module error in Elixir release

[almirsarajcic]

I'm deploying app using Elixir release and in production I was getting this error:

** (UndefinedFunctionError) function ESTree.Tools.Builder.identifier/1 is undefined (module ESTree.Tools.Builder is not available)
  ESTree.Tools.Builder.identifier(:Intercom)
  (intercom 0.0.1) lib/intercom.ex:21: Intercom.boot/1
  (intercom 0.0.1) lib/intercom.ex:16: Intercom.boot/1
  (intercom 0.0.1) lib/intercom.ex:58: Intercom.snippet/2

This PR fixes the error by defining estree dependency as a runtime to be included in the release.