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.

@Tapjoy/opsautomation @jjrussell