TST: add 32 bit travis tests

Implemented by setting up a chroot in the travis VM and moving all the
test logic, including bento build, to a single script.
This is still reasonable fast, about twice as slow as the normal tests.
When Travis updates to a newer OS it can be replaced by standard cross
compiling.
MAINT: enable verbose bento build
Merge pull request #4161 from juliantaylor/32bit-travis

TST: add 32 bit travis tests
Add '.FOR' to src_extensions

The uppercase source extension '.FOR' is also commonly used.
Merge pull request #4173 from leaz/patch-2

Add '.FOR' to src_extensions