Fix loading of gemspecs that contain unicode characters.

If Ruby's default encoding is not changed then the files are read in as
US-ASCII. When compared against the regular expression they were raising
encoding errors on 1.9 because the unicode characters could not be
converted to ASCII.

This changes the YAML header to check to match what is currently in 1.3
which doesn't have the problem (as it doesn't need to process the whole
string).

Should fix #2240. Possible make #2241 unneeded.