renames metadata[:example_group_block] -> metadata[:block]
renames metadata[:example_group_block] -> metadata[:block]

resolves deprecation warning:

Deprecation Warnings:

`metadata[:example_group_block]` is deprecated. Use `metadata[:block]` instead.
Called from ./spec_tests/spec/spec_helper.rb:18:in `yield'.

heartsucker/securedrop,jrosco/securedrop,chadmiller/securedrop,chadmiller/securedrop,jeann2013/securedrop,pwplus/securedrop,jaseg/securedrop,heartsucker/securedrop,heartsucker/securedrop,chadmiller/securedrop,pwplus/securedrop,jrosco/securedrop,jeann2013/securedrop,conorsch/securedrop,micahflee/securedrop,ehartsuyker/securedrop,chadmiller/securedrop,kelcecil/securedrop,kelcecil/securedrop,kelcecil/securedrop,heartsucker/securedrop,kelcecil/securedrop,pwplus/securedrop,harlo/securedrop,ageis/securedrop,pwplus/securedrop,ageis/securedrop,jaseg/securedrop,harlo/securedrop,garrettr/securedrop,chadmiller/securedrop,micahflee/securedrop,ehartsuyker/securedrop,micahflee/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop,conorsch/securedrop,kelcecil/securedrop,jaseg/securedrop,garrettr/securedrop,jaseg/securedrop,kelcecil/securedrop,jeann2013/securedrop,ageis/securedrop,jrosco/securedrop,jeann2013/securedrop,jrosco/securedrop,harlo/securedrop,conorsch/securedrop,jrosco/securedrop,jaseg/securedrop,garrettr/securedrop,jeann2013/securedrop,ehartsuyker/securedrop,pwplus/securedrop,jaseg/securedrop,pwplus/securedrop,ageis/securedrop,harlo/securedrop,jrosco/securedrop,ehartsuyker/securedrop,harlo/securedrop,ehartsuyker/securedrop,micahflee/securedrop,jeann2013/securedrop,harlo/securedrop,conorsch/securedrop,chadmiller/securedrop,heartsucker/securedrop