Use char literal instead of string literal with 1 char

In reality, this is a relatively small performance improvement,
especially with the previous improvements removing lots of superfluous
string handling, but still was measurable.

include/internal/catch_stream.hpp

@@ -61,7 +61,7 @@ namespace Catch {
         m_ofs.open( filename.c_str() );
         if( m_ofs.fail() ) {
             std::ostringstream oss;
-            oss << "Unable to open file: '" << filename << "'";
+            oss << "Unable to open file: '" << filename << '\'';
             throw std::domain_error( oss.str() );
         }
     }