[CI] Macos SDK installation fails on Travis

[asfimport]

See the failing build at https://travis-ci.org/apache/arrow/jobs/602560324

Pass -allowUntrasted flag during the installation.

Reporter: Krisztian Szucs / @kszucs

Related issues:

• [CI] Travis OSX failing to install sdk headers (duplicates)

_{Note: This issue was originally created as ARROW-6993. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
@kou this is causing the GLib tests to fail on master

[asfimport]

Kouhei Sutou / @kou:
This duplicates ARROW-6987.

[asfimport]

Todd Farmer / @toddfarmer:
Transitioning issue from Resolved to Closed to based on resolution field value.