Ignore ENV exported functions

We recently saw an issue where RVM versions 1.25.29-31 were exporting a
function into the environment using `export -f`. When Bash 4.3.27 was
released, it changed how this export was happening. This change broke
the environment parsing that we did in FPM for loading/saving our
.install-metadata file. This change will cause those exported functions
to be ignored.
Merge pull request #40 from gerbercj/fix/ignore_env_exported_functions

Ignore ENV exported functions