TST: skip floating point warning test failures on ARM.  Closes gh-413.
MAINT: remove painful to look at exec statement from np.errstate test.
TST: remove Python 2.4 and 2.5 testing from Travis config.

Done because for 1.8 we're not supporting these Python versions anymore,
and this PR break 2.4 compatibility.
Merge pull request #2879 from rgommers/issue-413-arm-failures

Fix ARM failures (gh-413)