do not pin deps to exact versions (#2844)

because
- they are locked in .lock file already
- individual dependencies can be updated with `dep ensure -update XXX`
- review process (and ^^^) should help us prevent accidental updates

Closes #2798

Gopkg.toml

@@ -20,57 +20,60 @@
 #   unused-packages = true
 #
 ###########################################################
-# NOTE: All packages should be pinned to specific versions.
-# Packages without releases must pin to a commit.
-
 
+# Allow only patch releases for serialization libraries
 [[constraint]]
-  name = "github.com/go-kit/kit"
-  version = "=0.6.0"
+  name = "github.com/tendermint/go-amino"
+  version = "~0.14.1"
 
 [[constraint]]
   name = "github.com/gogo/protobuf"
-  version = "=1.1.1"
+  version = "~1.1.1"
 
 [[constraint]]
   name = "github.com/golang/protobuf"
-  version = "=1.1.0"
+  version = "~1.1.0"
+
+# Allow only minor releases for other libraries
+[[constraint]]
+  name = "github.com/go-kit/kit"
+  version = "^0.6.0"
 
 [[constraint]]
   name = "github.com/gorilla/websocket"
-  version = "=1.2.0"
+  version = "^1.2.0"
 
 [[constraint]]
   name = "github.com/rs/cors"
-  version = "1.6.0"
+  version = "^1.6.0"
 
 [[constraint]]
   name = "github.com/pkg/errors"
-  version = "=0.8.0"
+  version = "^0.8.0"
 
 [[constraint]]
   name = "github.com/spf13/cobra"
-  version = "=0.0.1"
+  version = "^0.0.1"
 
 [[constraint]]
   name = "github.com/spf13/viper"
-  version = "=1.0.0"
+  version = "^1.0.0"
 
 [[constraint]]
   name = "github.com/stretchr/testify"
-  version = "=1.2.1"
-
-[[constraint]]
-  name = "github.com/tendermint/go-amino"
-  version = "v0.14.1"
+  version = "^1.2.1"
 
 [[constraint]]
   name = "google.golang.org/grpc"
-  version = "=1.13.0"
+  version = "~1.13.0"
 
 [[constraint]]
   name = "github.com/fortytw2/leaktest"
-  version = "=1.2.0"
+  version = "^1.2.0"
+
+[[constraint]]
+  name = "github.com/prometheus/client_golang"
+  version = "^0.9.1"
 
 ###################################
 ## Some repos dont have releases.
@@ -94,11 +97,6 @@
   name = "github.com/tendermint/btcd"
   revision = "e5840949ff4fff0c56f9b6a541e22b63581ea9df"
 
-# Haven't made a release since 2016.
-[[constraint]]
-  name = "github.com/prometheus/client_golang"
-  revision = "ae27198cdd90bf12cd134ad79d1366a6cf49f632"
-
 [[constraint]]
   name = "github.com/rcrowley/go-metrics"
   revision = "e2704e165165ec55d062f5919b4b29494e9fa790"