x/build: number of test failures

In lieu of an automated builder, I run the x/build tests in Travis CI. @andybons has fixed two problems with cmd/cl and with cloud.google.com depedencies, but this has revealed a number of underlying test failures:

--- FAIL: TestConnAgainstNetTest (2.03s)
	revdial_test.go:236: warning: the revdial's SetWriteDeadline support is not complete; some tests involving write deadlines known to be flaky
    --- PASS: TestConnAgainstNetTest/BasicIO (0.41s)
    --- PASS: TestConnAgainstNetTest/PingPong (0.19s)
    --- PASS: TestConnAgainstNetTest/RacyRead (0.34s)
    --- FAIL: TestConnAgainstNetTest/RacyWrite (0.42s)
    	testing.go:610: race detected during execution of test

TestCorpusCheck causes the entire Go corpus to download to the test instance, which eventually runs out of memory.

2017/10/13 18:26:13 Downloading 9495471 bytes of https://maintner.golang.org/logs/27 ...
2017/10/13 18:26:13 wrote /home/travis/.cache/golang-maintner/0027.growing.mutlog
fatal error: runtime: out of memory

Also, we need to update the go-github version for the gitmirror Docker file, I believe; IsPullRequest was added very recently.

src/golang.org/x/build/maintner/github.go:802: d.b.IsPullRequest undefined (type *github.Issue has no field or method IsPullRequest)
Removing intermediate container 65ca938e2dcb
The command '/bin/sh -c go install golang.org/x/build/cmd/gitmirror' returned a non-zero code: 2
make: *** [build0] Error 2