[PTFM-13697] Skip CentOS 5.11 in applet exec test

Summary:
Skip CentOS 5.11 when remotely running applets in
test_var_initialization, since 5.11's latest official OpenSSL package
is not sufficient for building/linking apps that target Ubuntu 12.04.

Using 'dx build --remote' is the preferred alternative to attempting to
cross-compile C++ apps on old distros that lack up-to-date dxcpp library
dependencies.

Test Plan: Ran test on CentOS 5, OS X 10.9, Ubuntu 12.04

Reviewers: psung

Reviewed By: psung

Subscribers: akislyuk

Differential Revision: https://phabricator.dnanexus.com/D176