Define toString template before using toString calls

A way to avoid (gcc 4.8.1) 
```
test/vector_tile.cpp:121:5:   required from here
test/internal/catch_tostring.hpp:105:35: error: ‘toString’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
             oss << toString( v[i] );
                                   ^
test/internal/catch_tostring.hpp:204:20: note: ‘std::string Catch::toString(unsigned char)’ declared here, later in the translation unit
 inline std::string toString( unsigned char value ) {
                    ^
```