Fix mktemp usage on Mac OS, and handling of environments where GPG is not available

[Daniel15]

Summary
Fixes two issues with the install script:

• Old versions of Mac OS (10.10 and below) require the -t option for mktemp
• The check for GPG wasn't returning correctly, and tried to run GPG even when not available

Test plan
mv /usr/bin/gpg /usr/bin/gpg-old and run install script. Ensure output looks like:

> WARNING: GPG is not installed, integrity can not be verified!
> Extracting to ~/.yarn...
> Adding to $PATH...

Move /usr/bin/gpg-old back to /usr/bin/gpg, ensure GPG is ran:

> Verifying integrity...
gpg: Signature made Mon 21 Nov 2016 10:18:40 PM STD using RSA key ID FD2497F5
gpg: Good signature from "Yarn Packaging <yarn@dan.cx>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 72EC F46A 56B4 AD39 C907  BBB7 1646 B01B 86E5 0310
     Subkey fingerprint: 6A01 0C51 6600 6599 AA17  F081 46C2 130D FD24 97F5
> GPG signature looks good
> Extracting to ~/.yarn...

Unfortunately I don't have any Mac OS 10.10 machines available to test it out, but I think this will work as expected.

cc @BanzaiMan

Closes #2012
Closes #1984