Silence type punning warning reported by gcc 6.5

[kevingranade]

Summary

SUMMARY: None

Purpose of change

Travis builds under GCC 6.5 have been failing due to a type punning warning.
https://travis-ci.org/CleverRaven/Cataclysm-DDA/jobs/619431840

Describe the solution

Silence this class of warnings when building under anything from the GCC 6 family.

Describe alternatives you've considered

There are new versions of colony that may have addressed this, but I don't want to re-import right now.

Testing

Ideally reproduce and test this on a GCC 6.5 system, but IDK how straightforward that will be.

Additional context

Text of the warning:

In file included from colony_test.cpp:8:0:
../src/colony.h: In instantiation of ‘void cata::colony<element_type, element_allocator_type, element_skipfield_type>::fill_skipblock(const element_type&, cata::colony<element_type, element_allocator_type, element_skipfield_type>::aligned_pointer_type, cata::colony<element_type, element_allocator_type, element_skipfield_type>::skipfield_pointer_type, cata::colony<element_type, element_allocator_type, element_skipfield_type>::skipfield_type) [with element_type = int; element_allocator_type = std::allocator; element_skipfield_type = short unsigned int; cata::colony<element_type, element_allocator_type, element_skipfield_type>::aligned_pointer_type = std::aligned_storage<4ul, 4ul>::type*; cata::colony<element_type, element_allocator_type, element_skipfield_type>::skipfield_pointer_type = short unsigned int*; cata::colony<element_type, element_allocator_type, element_skipfield_type>::skipfield_type = short unsigned int]’:

../src/colony.h:1555:43: required from ‘void cata::colony<element_type, element_allocator_type, element_skipfield_type>::insert(cata::colony<element_type, element_allocator_type, element_skipfield_type>::size_type, const element_type&) [with element_type = int; element_allocator_type = std::allocator; element_skipfield_type = short unsigned int; cata::colony<element_type, element_allocator_type, element_skipfield_type>::size_type = long unsigned int]’

colony_test.cpp:689:39: required from here

../src/colony.h:1463:34: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]

                              *( reinterpret_cast<aligned_pointer_type>( &aligned_copy ) ) );

                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~