Bumps vagrant version version used in snapci pipeline.
Bumps vagrant version version used in snapci pipeline.

Iterating 1.7.2 -> 1.7.3, conservatively. If no problems,
can try again on 1.7.4.

heartsucker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,pwplus/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,garrettr/securedrop,pwplus/securedrop,micahflee/securedrop,heartsucker/securedrop,heartsucker/securedrop,pwplus/securedrop,conorsch/securedrop,micahflee/securedrop,ehartsuyker/securedrop,pwplus/securedrop,micahflee/securedrop,ageis/securedrop,ageis/securedrop,garrettr/securedrop,ageis/securedrop,garrettr/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop,ageis/securedrop,heartsucker/securedrop,pwplus/securedrop,conorsch/securedrop,pwplus/securedrop