Added const to eliminate warning

When building with gcc I get the following warning in several places,

warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]

This happens because there are string literals being passed to readScriptValue
as char*. The fix for this is to change the signature of readScriptValue to take
const char*, which is consistent with strstr() where the variable is used, and
eliminates the compiler warning.