Update the minimum Numpy version to 1.5.

Remove workarounds and "knownfail" tests for Numpy 1.4.

Add an explicit check for the correct version of Numpy on importing astropy.
Use a substitution for the Numpy version so we don't have to remember to
update it in multiple places.
Remove another Numpy 1.4 workaround
Fix Numpy 1.5 compatibility.
Merge pull request #1479 from mdboom/misc/remove-numpy-14

Warning if Numpy < 1.5 is used