update levigo to 1.0.0 (#3389)

Although the version we were pinning to is from Nov. 2016 there were no substantial changes:

    jmhodges/levigo@2b8c778 added go-modules support (no code changes)
    jmhodges/levigo@853d788 added a badge to the readme

closes #3381

Gopkg.lock

@@ -154,11 +154,12 @@
   version = "v1.0"
 
 [[projects]]
-  digest = "1:39b27d1381a30421f9813967a5866fba35dc1d4df43a6eefe3b7a5444cb07214"
+  digest = "1:a74b5a8e34ee5843cd6e65f698f3e75614f812ff170c2243425d75bc091e9af2"
   name = "github.com/jmhodges/levigo"
   packages = ["."]
   pruneopts = "UT"
-  revision = "c42d9e0ca023e2198120196f842701bb4c55d7b9"
+  revision = "853d788c5c416eaaee5b044570784a96c7a26975"
+  version = "v1.0.0"
 
 [[projects]]
   branch = "master"

Gopkg.toml

@@ -75,12 +75,15 @@
   name = "github.com/prometheus/client_golang"
   version = "^0.9.1"
 
+[[constraint]]
+  name = "github.com/jmhodges/levigo"
+  version = "^1.0.0"
+
 ###################################
 ## Repos which don't have releases.
 
 ## - github.com/btcsuite/btcd
 ## - golang.org/x/crypto
-## - github.com/jmhodges/levigo
 ## - github.com/btcsuite/btcutil
 ## - github.com/rcrowley/go-metrics
 ## - golang.org/x/net