utils: create GEM_HOME when installing Gems.

It may not exist before Gem installation which means that the resulting
installed gem will not be found in the PATH.

Refactor PATH generation.

caveats: tweak keg_only wording

Current:
```
This formula is keg-only, which means it was not symlinked into /usr/local.

Qt has CMake issues when linked
```

After:
```
This formula is keg-only, which means it was not symlinked into /usr/local,
because Qt has CMake issues when linked.
```

Revert "Fix `IO#select` blocking."

audit: formally ban go get usage

pull: fix status code check in check_bintray_mirror

The status code of the last redirect should be 2xx to be deemed successful.

Update `plist` to 3.3.0.

There's been an informal ban for a while but let's
be punchier because this crops up still.

audit: tweak dependency option audit.

Make sure that `if build.with?` isn't caught.

docs: tweak CSS.

Import new brew.sh CSS changes to improve the appearance of tables and
links.

tap-new: bump Xcode to 8.3.

brew.sh: tweak macOS user agent again.