Bump github.com/imdario/mergo reference to v0.3.7 (#135)

For somehow reason, current glide.lock pinned `mergo` to a very old
version (dates back to 2014), while github.com/Masterminds/sprig
referenced asked for ~v0.3.7 in their glide.yaml which caused build
failure.

This change bumps Mergo to v0.3.7 to satisfy the dependency requirement

Fixes #134

Signed-off-by: Qiu Yu <[email protected]>

Author: unicell