Fix for -Wliteral-suffix C++11 warning

In C++11, concatenation of literal and preprocessor strings requires
an intervening space so as to avoid conflicting with the new string
suffix syntax.  g++ seems to be only warning about this for now, but
let's fix it before they turn it into an error.
Merge pull request #263 from libMesh/Wliteral-suffix

Fix for -Wliteral-suffix C++11 warning