TST: decrease test tolerance of interpolate test.

Fixes the following failure on 32-bit Linux, Bento build:

    ======================================================================
    FAIL: test_fitpack.TestSplder.test_splder_vs_splev
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/home/rgommers/.local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
        self.test(*self.arg)
      File "/home/rgommers/Code/scipy/scipy/interpolate/tests/test_fitpack.py", line 304, in test_splder_vs_splev
        assert_allclose(dy, dy2)
      File "/home/rgommers/Code/numpy/numpy/testing/utils.py", line 1181, in assert_allclose
        verbose=verbose, header=header)
      File "/home/rgommers/Code/numpy/numpy/testing/utils.py", line 644, in assert_array_compare
        raise AssertionError(msg)
    AssertionError:
    Not equal to tolerance rtol=1e-07, atol=0

    (mismatch 100.0%)
     x: array([-3980.01505714, -3968.01797701, -3956.03903697, ..., -4049.189339  ,
           -4060.77715859, -4072.38145423])
     y: array([-3980.01513343, -3968.01803805, -3956.039098  , ..., -4049.189339  ,
           -4060.77715859, -4072.38145423])
Merge pull request #2870 from rgommers/fitpack-failure

TST: interpolate: decrease test tolerance of splder test