chruby-exec: silence messages like "Using ruby-1.9.3"

When I ran the tests for chruby on your commit 2dcf2435dd using `make test`, one of the tests of chruby-exec fails:

    >>> Running ./test/chruby_exec_test.sh ...
    test_chruby_exec_no_arguments
    test_chruby_exec_no_command
    test_chruby_exec
    ASSERT:did change the ruby expected:<1.9.3> but was:<Using ruby-1.9.3
    1.9.3>

The code for that test is:

    test_chruby_exec()
    {
    	local command="ruby -e 'print RUBY_VERSION'"
    	local ruby_version=$(chruby-exec "$TEST_RUBY_VERSION" -- $command)
    
    	assertEquals "did change the ruby" "$TEST_RUBY_VERSION" "$ruby_version"
    }

Apparently, this test is expecting chruby-exec NOT to print out a "Using ruby-1.9.3" line but it did.  That behavior seems a bit annoying to me.  Generally you want your cron jobs to be silent unless there is an error.  I chose to fix chruby-exec to conform to this test.  The way I did that was to pipe the standard output of chruby into `/dev/null`:

    exec $SHELL -l -i -c "chruby $argv >/dev/null && $*"