Loosen tesla dep

[matthewoden]

Failed to use "tesla" because
  libvault (version 0.1.2) requires ~> 1.0.0
  mix.exs specifies ~> 1.2.1

** (Mix) Hex dependency resolution failed, change the version requirements of your dependencies or unlock them (by using mix deps.update or mix deps.unlock). If you are unable to resolve the conflicts you can try overriding with {:dependency, "~> 1.0", override: true}