Fix for resolving variable references

[ombran]

When I use a variable in a custom property of serverless.yml, its value was not resolved.
For example, (this environment variable could not use.)

custom:
  prune:
    automatic: true
    number: ${env:ENV_PRUNE_NUMBER}

Since the variable is not resolved in the constructor, it must be acquired by hook side. So I Fixed.
( FYI: https://forum.serverless.com/t/serverless-plugin-resolving-variable-references/2233 )

[coveralls]

Coverage increased (+0.1%) to 92.958% when pulling 01ad168 on ombran:master into 86455c1 on claygregory:master.

[claygregory]

Thanks @ombran! Merged and released as v1.3.1