Migrate to dep

Ran `dep init` and then `dep ensure`. There were no changes in `vendor/`.

Uses the latest stable release of `dep`:
```
$ dep version
dep:
 version     : v0.4.1
 build date  : 2018-01-27
 git hash    : 37d9ea0
 go version  : go1.9.3
 go compiler : gc
 platform    : darwin/amd64
```

Resolves s12v#9

.travis.yml

@@ -5,8 +5,8 @@ go:
 
 install:
   - go get github.com/elastic/beats || true
-  - curl https://glide.sh/get | sh
-  - glide install
+  - curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
+  - dep ensure
 
 script:
   - |

Gopkg.toml

@@ -0,0 +1,39 @@
+# Gopkg.toml example
+#
+# Refer to https://github.com/golang/dep/blob/master/docs/Gopkg.toml.md
+# for detailed Gopkg.toml documentation.
+#
+# required = ["github.com/user/thing/cmd/thing"]
+# ignored = ["github.com/user/project/pkgX", "bitbucket.org/user/project/pkgA/pkgY"]
+#
+# [[constraint]]
+#   name = "github.com/user/project"
+#   version = "1.0.0"
+#
+# [[constraint]]
+#   name = "github.com/user/project2"
+#   branch = "dev"
+#   source = "github.com/myfork/project2"
+#
+# [[override]]
+#   name = "github.com/x/y"
+#   version = "2.4.0"
+#
+# [prune]
+#   non-go = false
+#   go-tests = true
+#   unused-packages = true
+
+ignored = ["github.com/elastic/beats"]
+
+[[constraint]]
+  name = "github.com/aws/aws-sdk-go"
+  version = "1.12.53"
+
+[[constraint]]
+  name = "github.com/elastic/beats"
+  version = "6.2.4"
+
+[prune]
+  go-tests = true
+  unused-packages = true

README.md

@@ -44,8 +44,8 @@ Build requires Go 1.10+ and Linux. You need to define Filebeat version (`v6.1.3`
 
 ```
 go get github.com/elastic/beats
-# curl https://glide.sh/get | sh
-glide install
+# curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
+dep ensure
 make BEATS_VERSION=v6.1.3
 ```