vendor: Update github.com/gogo/protobuf

Also tweaks the proto definitions:

 - [packed=false] on the block_indexes field to retain compat with
   v0.14.16 and earlier.

 - Uses the vendored protobuf package in include paths.

And, "build.go setup" will install the vendored protoc-gen-gogofast.
This should ensure that a proto rebuild isn't so dependent on whatever
version of the compiler and package the developer has installed...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3864

vendor/github.com/gogo/protobuf/.travis.yml

@@ -0,0 +1,25 @@
+env:
+  - PROTOBUF_VERSION=2.6.1
+  - PROTOBUF_VERSION=3.0.2
+  - PROTOBUF_VERSION=3.1.0
+
+before_install:
+  - ./install-protobuf.sh
+  - PATH=/home/travis/bin:$PATH protoc --version
+
+script:
+  - PATH=/home/travis/bin:$PATH make buildserverall
+  - echo $TRAVIS_GO_VERSION
+  - if [ "$TRAVIS_GO_VERSION" == 1.7.1 ] && [[ "$PROTOBUF_VERSION" == 3.1.0 ]]; then ! git status --porcelain | read || (git status; git diff; exit 1); fi
+
+language: go
+
+go:
+  - 1.5.4
+  - 1.6.3
+  - 1.7.1
+
+matrix:
+  allow_failures:
+    - go: 1.5.4
+    - go: 1.6.3