Make string encoding explitic

Ruby 2.0 enforces explicit string encodings. The char_constant
testcase fails because the internal (SWIG_FromCharPtrAndSize, using
rb_str_new) defaults to ASCII-8BIT while the test-suite file defaults
to the current shell LOCALE setting.

This patch sets the char_constant_runme.rb encoding to ASCII-8BIT.
Merge pull request #117 from kkaempf/ruby-strict-encoding

Make string encoding explitic