Avoid passing more than one instance of ERL_LIBS to child processes

This commit fixes issue #98 by removing the ERL_LIBS and REBAR_DEPS_DIR
from the list of environment variables exported by the rebar_port_compiler
plugin.
Merge pull request #98 from jcomellas/jc-no-erl-libs-repetition

Repetition of environment variable definitions in child processes (ports)