Fix unsafe variables

[mspanc]

When I am using toniq 1.2.0 with elixir 1.3.4, the following warning appears during compilation:

remote: warning: the variable "v1" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:
remote:
remote:     case int do
remote:       1 -> atom = :one
remote:       2 -> atom = :two
remote:     end
remote:
remote: should be written as
remote:
remote:     atom =
remote:       case int do
remote:         1 -> :one
remote:         2 -> :two
remote:       end
remote:
remote: Unsafe variable found at:
remote:   lib/toniq/job.ex:28