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.
erlc: fix comment

test_compile/3 is used by eunit and qc, but the comment was only
referring to 'rebar eunit'.
Fix #220 (Reported-by: Joseph Norton)

When running 'rebar qc' or 'rebar eunit', we were erroneously fetching
erl_opts more than once.
Merge pull request #98 from jcomellas/jc-no-erl-libs-repetition

Repetition of environment variable definitions in child processes (ports)
Merge pull request #234 from tuncer/fix-220

Fix #220 (Reported-by: Joseph Norton)