Add explicit library dependencies

Adds Gopkg.toml and Gopkg.lock with initial dependency
versions. Includes workaround for Apache Thrift repo migration.
Ignores the vendor folder and adds a travis test to ensure dependency
version updates don't break the build.

Fixes census-instrumentation#684.

.gitignore

@@ -3,3 +3,5 @@
 # go.opencensus.io/exporter/aws
 /exporter/aws/
 
+# Ignore Go vendor directory
+/.vendor/

.travis.yml

@@ -15,6 +15,8 @@ before_script:
   - PKGS=$(go list ./... | grep -v /vendor/)             # All the import paths, excluding vendor/ if any
 
 script:
+  - dep ensure -v
+  - go build ./..                     # Ensure dependency updates don't break build
   - if [ -n "$(gofmt -s -l .)" ]; then echo "gofmt the following files:"; gofmt -s -l .; exit 1; fi
   - go vet ./...
   - go test -v -race $PKGS            # Run all the tests with the race detector enabled

Gopkg.toml

@@ -0,0 +1,44 @@
+[[constraint]]
+  name = "cloud.google.com/go"
+  version = "0.21.0"
+
+[[constraint]]
+  branch = "master"
+  name = "git.apache.org/thrift.git"
+  source = "github.com/apache/thrift"
+
+[[constraint]]
+  name = "github.com/golang/protobuf"
+  version = "1.0.0"
+
+[[constraint]]
+  name = "github.com/openzipkin/zipkin-go"
+  version = "0.1.0"
+
+[[constraint]]
+  name = "github.com/prometheus/client_golang"
+  version = "0.8.0"
+
+[[constraint]]
+  branch = "master"
+  name = "golang.org/x/net"
+
+[[constraint]]
+  branch = "master"
+  name = "golang.org/x/oauth2"
+
+[[constraint]]
+  branch = "master"
+  name = "google.golang.org/api"
+
+[[constraint]]
+  branch = "master"
+  name = "google.golang.org/genproto"
+
+[[constraint]]
+  name = "google.golang.org/grpc"
+  version = "1.11.3"
+
+[prune]
+  go-tests = true
+  unused-packages = true