Update Travis CI to use standard test script.

This also: removes old go versions; disables sudo; turns off email
notifications and caches gx directories; disables "Non-standard
signatures" check in "go vet" (otherwise "go vet" will complain about
the signature for ReadFrom and WriteTo methods for Chan).

.travis.yml

+sudo: false
+
 language: go
 
 go:
-  - 1.3
-  - 1.4
-  - release
+  - 1.9.x
+
+install:
+  - make deps
 
 script:
-  - go test -race -cpu=5 -v ./...
+  - bash <(curl -s https://raw.githubusercontent.com/ipfs/ci-helpers/master/travis-ci/run-standard-tests.sh)
+
+cache:
+  directories:
+    - $GOPATH/src/gx
+
+notifications:
+  email: false
+
+env: GOTFLAGS="-race -cpu=5" GOVETCMD="go vet -methods=false"

Makefile

+gx:
+	go get github.com/whyrusleeping/gx
+	go get github.com/whyrusleeping/gx-go
+
+deps: gx
+	gx --verbose install --global
+	gx-go rewrite
+
+publish:
+	gx-go rewrite --undo
+

codecov.yml

+coverage:
+  range: "50...100"
+comment: off