Auto merge of #48891 - alexcrichton:dist-osx-9.3, r=kennytm

travis: Upgrade dist builders for OSX

This commit upgrades the dist builders for OSX to Travis's new `xcode9.3-moar`
image which has 3 cores available to it instead of 2. This should help us
provide speedier builds on OSX and hit timeouts less in theory!

Note that historically the dist builders for OSX have been a different version
than the ones that are running tests. I had forgotten why this was the case and
digging around brought up 307615567 where apparently Xcode 8 wasn't able to
compile LLVM with `MACOSX_DEPLOYMENT_TARGET=10.7` which we desired. On a whim I
gave this PR a spin and it [looks like][green] this has since been fixed (maybe
in LLVM?). In any case those green builds should hopefully mean that we can
safely upgrade and get faster infrastructure to boot.

This commit also includes an upgrade of OpenSSL. This is not done for security
reasons but rather build system reasons. Originally builds with the new image
[did not succeed][red] due to weird build failures in OpenSSL, but upgrading
seems to have made the spurious errors go away to here's to also hoping that's
fixed!

[green]: https://travis-ci.org/rust-lang/rust/builds/351353412
[red]: https://travis-ci.org/rust-lang/rust/builds/350969248

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