.travis.yml: specify distribution

Change-Id: I046ede4e1062553c5b4bb496b983c3f206f8485e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/167277
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

.gitignore

@@ -1,20 +1,3 @@
 .cache
 vendor
-
-.DS_Store
-*.[568ao]
-*.ao
-*.so
-*.pyc
-._*
-.nfs.*
-[568a].out
-*~
-*.orig
-core
-_obj
-_test
-_testmain.go
-
-# Conformance test output and transient files.
-conformance/failing_tests.txt
+cmd/protoc-gen-go/protoc-gen-go

.travis.yml

@@ -1,3 +1,4 @@
+dist: xenial
 script:
   - ./test.bash
 cache: