Fix ruby RbConfig deprecation warning

Vim displays a warning when saving c files:
E492: Not an editor command:  -I-e:1: Use RbConfig instead of obsolete and deprecated Config./Users/andrew/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/x86_64-darw
in12.0.0^@ -I-e:1: Use RbConfig instead of obsolete and deprecated Config./Users/andrew/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/x86_64-darwin12.0.0^@ -I-e:1:
Use RbConfig instead of obsolete and deprecated Config./Users/andrew/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/x86_64-darwin12.0.0^@ -I-e:1: Use RbConfig instea
d of obsolete and deprecated Config./Users/andrew/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/x86_64-darwin12.0.0^@