ENH: Check that there are sufficient dof for estimation.
BUG: Never squeeze to a scalar.
ENH: Handle rare case of nobs == 1.
BUG: Don't let maxlag > nobs. Closes #1146.
TST: Add test for #1146.
Merge pull request #1149 from jseabold/fix-1146

BUG: Fix small data issues for ARIMA.