Update toml dep to fetch its latest master branch instead of v0.3.0

toml v0.3.0 was released in March 2017 and is missing few fixes made in toml
package after the fact. So use the master branch instead.

This fixes a regression when switch was made from govendor vendor.json dep
management to Go dep.

Fixes #3998

Gopkg.toml

@@ -2,7 +2,7 @@
 
 [[constraint]]
   name = "github.com/BurntSushi/toml"
-  version = "0.3.0"
+  branch = "master"
 
 [[constraint]]
   name = "github.com/PuerkitoBio/purell"