Specify newer go version for testing
Specify newer go version for testing

Trying to fix the following TravisCI test error:

--- FAIL: TestPostMessageError-2 (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x4615ab]
goroutine 37 [running]:
testing.func·006()
	/usr/local/go/src/testing/testing.go:441 +0x181
github.com/yinghau76/phabricator-to-slack.(*Slack).PostMessage(0xc2081a71d0, 0x751030, 0x4, 0x782370, 0x10, 0x0, 0x0)
	github.com/yinghau76/phabricator-to-slack/_test/_obj_test/slack.go:49 +0x68b
github.com/yinghau76/phabricator-to-slack.TestPostMessageError(0xc208076ab0)
	/home/travis/gopath/src/github.com/yinghau76/phabricator-to-slack/slack_test.go:51 +0x19c
testing.tRunner(0xc208076ab0, 0x8da838)
	/usr/local/go/src/testing/testing.go:447 +0xbf
created by testing.RunTests
	/usr/local/go/src/testing/testing.go:555 +0xa8b

yinghau76/phabricator-to-slack