Update GC-related comments in dense_map and dense_map_pair

This removes the nm_register_values and nm_unregister_values calls completely
and adds a note for future reference about what the volatile is doing there.
Trying out forcing Travis to use GCC 4.7
Added range checking to #yale_nd_row
Merge pull request #145 from cjfuller/volatile_doc

Update GC-related comments in dense_map and dense_map_pair
Possibly fixed bug in get_version_info, for writing and reading files.
Merge branch 'master' of github.com:SciRuby/nmatrix