(#6928) backport Symbol#to_proc for Ruby < 1.8.7

We use the &:foo symbol-to-proc syntax in some of our code, so to avoid
problems on Ruby earlier than 1.8.7 we should backport the support in our
monkey-patch file.

Backported to 2.6.x to ease working there as part of #2848.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>