Previously GsonHttpMessageConvertersConfiguration was unconditional
with its nested configuration class being
@ConditionalOnClass(Gson.class). This led to the unnecessary
registration of the GsonHttpMessageConvertersConfiguration bean when
Gson isn't on the classpath.

This commit moves the condition up onto the outer class so that no
Gson-related beans are created when Gson is not on the classpath.

Closes gh-2778