Update vendors + allow default_ttl to be set to 0

[aerostitch]

As described in #20 and hashicorp/terraform#13648 and hashicorp/terraform#12910, not being able to set the default_ttl to 0 is quite an unexpected behavior.
To be able to do that we have to update the github.com/ajg/form dependency (I used make updatedeps so all have been updated, tested and had to modify fastly_test.go to fix an incompatibility), which is the 1st commit of this PR.

The 2nd removes the omitempty which removes the parameter when set to 0 and change the form encoder to keep zero values. Added a test for it.
Let me know if that's acceptable.

Thanks!

[aerostitch]

Weird, the tests don't fail in go 1.8 (including the fix proposed in #39):

$ make test testrace
==> Generating...
==> Running tests...
ok  	github.com/sethvargo/go-fastly	0.436s
==> Running tests (race)...
ok  	github.com/sethvargo/go-fastly	2.266s

Will dig.

[aerostitch]

That was due to go-cleanhttp dropping compatibility with go 1.6 in hashicorp/go-cleanhttp@a459706
Kept that out of the dependencies update.
The test work with 1.6 too now.